A five-minute walk from the rickety, raised track that carries the 5 train through the Bronx, the English teacher Argos Gonzalez balanced a rounded metal bowl on an outstretched palm. His class—a mix of black and Hispanic students in their late teens, most of whom live in one of the poorest districts in New York City—by now were used to the sight of this unusual object: a Tibetan meditation bell.

“Today we’re going to talk about mindfulness of emotion,” Gonzalez said with a hint of a Venezuelan accent. “You guys remember what mindfulness is?” Met with quiet stares, Gonzalez gestured to one of the posters pasted at the back of the classroom, where the students a few weeks earlier had brainstormed terms describing the meaning of “mindfulness.” There were some tentative mumblings: “being focused,” “being aware of our surroundings.”

Gonzalez nodded. “Right. But it’s also being aware of our feelings, our emotions, and how they impact us.”

Arturo A. Schomburg Satellite Academy is what is known in New York City as a transfer school, a small high school designed to re-engage students who have dropped out or fallen behind. This academy occupies two floors of a hulking, grey building that’s also home to two other public schools. For the most part, Gonzalez told me, the kids who come here genuinely want to graduate, but attendance is their biggest barrier to success. On the day I visited, one of Gonzalez’s students had just been released from jail; one recently had an abortion; one had watched a friend bleed to death from a gunshot wound the previous year. Between finding money to put food on the table and dealing with unstable family members, these students’ minds are often crowded with concerns more pressing than schoolwork.

Still holding the bowl, Gonzalez continued with the day’s lesson. “I’m going to say a couple of words to you. You’re not literally going to feel that emotion, but the word is going to trigger something, it’s going to make you think of something or feel something. Try to explore it.”

The slightly built, 30-something Gonzalez, who wears a wide smile and a scruffy beard, first learned about mindfulness from his wife, a yoga teacher in schools around the city. His students referred to him by his first name, and Gonzalez addressed them just as informally—greeting them in the morning with a high five and a “Sup,” or “How you doing, bro?” or even “Hey, mamma.” He told me he strives to make school relevant—explaining what a “motif” is by comparing it to the hook of a rap song, for example—and believes in the value of hands-on teaching, emailing students individually to check in when they don’t show up.

“First, sit up straight, put your feet flat on the ground. Let your eyes close.” Gonzalez demonstrated as he instructed. Most of the 15 or so students followed suit—though a few scribbled surreptitiously to finish overdue assignments. Gonzalez tapped the bowl and a rich, metallic sound rang out. The class fell quiet as the note reverberated.

“Take a deep breath into your belly. As you breathe in and breathe out, notice that your breath is going to be stronger in a certain part of your body. Maybe it’s your belly, your chest, or your nose. We’ll begin with trying to count to 10 breaths.”

There was silence but for the hiss of the 5 train pulling into the station, the clunk of garbage cans, the faint siren of a police car.

“If you get lost in thought, it’s okay. Just come back and count again. Whether you get up to 10 or not doesn’t really matter. It’s just a way to focus [your] mind.”

* * *

It may not be the typical way to start an English class, but Gonzalez’s students were familiar with these five-minute mindfulness exercises—from counting breaths and focusing on the sensations of breathing, to visualizing thoughts and feelings—that he uses to help train their attention, quiet their thoughts, and regulate their emotions.

Jon Kabat-Zinn, the biologist who first coined the term “mindfulness” in the ’70s, defines it as a state of mind: the act of “paying attention on purpose” to the present moment, with a “non-judgmental” attitude. But mindfulness is really a secular philosophy and set of techniques adapted from thousands-of-years-old Buddhist meditation traditions—ones that only recently landed in mainstream Western consciousness. It was Kabat-Zinn who first formally brought mindfulness into a medical setting; he developed the Mindfulness-Based Stress Reduction (MBSR) program, which used specific exercises to help patients dealing with chronic pain and is now widely applied in other therapeutic contexts, and founded the Center for Mindfulness at UMass Medical School.

Mindfulness has also been harnessed in increasingly diverse contexts beyond health care—some uses more legitimate than others. Last year, the Congressman Tim Ryan introduced mindfulness into weekly staff meetings on the Hill, Time published a cover story on the topic, and CNN’s Anderson Cooper dedicated a primetime segment to showcasing his own transformative experience at a mindfulness retreat center. Google, General Mills, the Seattle Seahawks, and the U.S. military have all embraced mindfulness as a means of boosting performance and productivity, while its potential as an antidote to the distractions and stress of everyday life is increasingly promoted within the general population and has spurred a cottage industry of books, magazines, and smartphone apps.

Even when the concept of mindfulness was created it was slightly nebulous; now, as it is reappropriated and circulated in the media, it has become even more so. The lack of a universal definition for mindfulness, along with its increasing association with celebrity and vague implications of spirituality, health, and happiness leave some skeptics dismissing it as a superficial, hokey fad. Meanwhile, practicing Buddhists and others who believe strongly in the spiritual roots of mindfulness are concerned that the meditation techniques are being poorly adopted without a proper understanding of the principles behind them, and the long-term commitment they require—a phenomenon they call “McMindfulness.”

Still, the body of scientific research illustrating the positive effects of mindfulness training on mental health and well-being—at the level of the brain as well as at the level of behavior—grows steadily more well-established: It improves attention, reduces stress, and results in better emotional regulation and an improved capacity for compassion and empathy. Brain-imaging studies at Harvard and Mass General Hospital have shown that long-term mindfulness training can help thicken the cortical regions related to attention and sensory processing, and may offset thinning of those areas that typically comes with aging. Mindfulness is widely considered effective in psychotherapy as a treatment not just for adults, but also for children and adolescents with aggression, ADHD, or mental-health problems like anxiety. (It remains to be seen whether mindfulness alone is a sufficient replacement for other therapies. In a review last year of 47 different randomized clinical trials, The Journal of the American Medical Association suggested that mindfulness training wasn’t any more effective than other types of therapy, like drugs.)

This strong base of research, along with a growing body of supporters, are fueling the momentum behind mindfulness. And as Gonzalez demonstrates, it’s now spreading to schools, where it could potentially have an impact on students’ well-being; a quarter of American adolescents suffer from a mental disorder, according to a 2010 Johns Hopkins study.

The first major effort to use mindfulness in schools began in the UK in 2007 with a series of fixed lesson plans delivered in classrooms across the country. Interest in the movement has picked up pace since. This past July, Oxford researchers announced plans to launch a large-scale, seven-year, $10 million study on mindfulness in education next year. More than a dozen similar initiatives have sprouted in the U.S., grassroots programs that train teachers in mindfulness and generate their own curricula. Among the two largest are MindUP and Mindful Schools, the California-based nonprofit that trained Gonzalez, which continue to spearhead the country’s steadily growing, but piecemeal, mindfulness-in-education movement. Since its founding in 2010, Mindful Schools has trained thousands of teachers through its online programs, most of them in California, New York, and Washington, D.C., who are said to have a total reach of 300,000 students.

After Gonzalez and his wife signed up for one of their six-week courses—Mindful Schools’ training is open to any educator or mental-health professional who wants to teach mindfulness to young people—he was able to convince his school administrators to help him pay for a year-long certification program. Through a series of online lectures, weekly breakout sessions, monthly meetings, and two week-long summer retreats, Gonzalez worked on his own mindfulness skills, honing his ability to control his attention and regulate his own emotions while receiving specific guidance on how to teach those same skills to the youth populations he’d be working with. Gonzalez also received training about the biology of the nervous system, child development, and the neuroscientific basis for mindfulness’s effects.

Not all mindfulness programs are in schools like Gonzalez’s, where large numbers of students have been identified as disordered or disruptive, or struggle with mental-health problems and unstable living situations. Middlesex School, a prestigious boarding school in Massachusetts, requires that all incoming freshmen take a mindfulness course. The program, which was founded by an alumnus who used mindfulness to cope with both sports-related performance anxiety and T-Cell lymphoma, has proven popular among students. A vast majority—97 percent—of students surveyed in 2014 said they would recommend the course to others, reporting benefits ranging from better sleep and diminished stress to increased focus on schoolwork.

Education reformers have long maintained that there is a fundamental connection between emotional imbalance and poor life prospects. As Paul Tough argued and popularized in How Children Succeed, stress early in life can prompt a cascade of negative effects, psychologically and neurologically—poor self-control and underdeveloped executive function, in particular. The U.S. education system’s focus on cognitive intelligence—IQ scores and academic skills like arithmetic—undermines the development of equally vital forms of non-cognitive intelligence. This type of intelligence entails dimensions of the mind that are difficult to quantify: It is the foundation of good character, resilience, and long-term life fulfillment. It is this part of the mind that mindfulness seeks to address.

* * *

Efforts to implement mindfulness in classrooms haven’t always gone smoothly. Some parents and administrators have challenged its use in schools based on its religious roots—and in at least one instance even managed to shut a program down. As mindfulness is used more routinely in the medical sphere, these belief-based critiques are becoming less common. But the lack of evidence demonstrating the long-term academic impact of mindfulness has raised concerns about its role as an educational tool. Given the inherent nebulousness of mindfulness as a concept, and the grassroots status of the movement, these concerns are understandable.

Qualitative evidence touting the benefits of mindfulness in the classroom—like Mindful Schools’ encouraging survey results and uplifting anecdotes from participants—is easy to come by, and several short-term research studies on elementary- and middle-school students have shown positive results. But serious questions remain about the overall efficacy of such programs on non-subjective measurements of well-being and academic performance, such as test scores, graduation rates, mental-health referrals, and overall life outcomes.

The lack of rigorous, robust, and long-term studies on mindfulness is what makes people like the Penn State University psychologist Mark Greenberg cautious. Greenberg works with the Collaborative for Academic, Social, and Emotional Learning—one of the groups at the forefront of the two-decade-old social-and-emotional-learning (SEL) movement. The mindfulness-in-education movement has a lot in common with, and in many ways complements, SEL, since both aim to teach children how to build self-awareness, effectively handle their emotions, and empathetically manage their relationships. Unlike mindfulness, however, which takes more of an inside-out approach by helping students to slow down, intentionally focus their attention from moment to moment, and build compassion, SEL works from the outside in, teaching children a set of skills like how to mediate a conflict, or how to verbally express and explain their emotions to improve communication. Research shows that SEL programs alone have boosted kids’ academic performance, as well as benefitting them socially and emotionally—but many believe mindfulness should also belong in the SEL toolkit.

Linda Lantieri, who helped found the SEL collaborative and has been working on these issues for decades, argues that the best approach to education combines mindfulness and SEL skills rather than treating one as a sufficient replacement for the other. While Greenberg agrees with Lantieri, he is a sober voice amidst the hype and enthusiasm about mindfulness, earning him the fond title of “curmudgeon” in some circles. “We don’t know if these effects last,” Greenberg told me. “Right now the promised benefits far exceed the actual findings.” He is also concerned that mindfulness is just one “flavor of the month” that may detract attention from SEL programs supported by more substantial evidence. (Greenberg recently co-authored an impressive longitudinal study that followed hundreds of students as they progressed from early childhood through young adulthood and found that poor social-emotional skills in kindergarten helped predict negative outcomes across multiple domains of education, employment, criminal activity, substance use, and mental health.)

Mindful Schools is aware of these criticisms, and is beefing up its research efforts. In 2012, the group worked with a University of California at Davis research team to conduct a randomized controlled study of three elementary schools in Oakland, California. They sent in outside mindfulness instructors for 15 minutes, three times a week, to teach some classrooms but not others, and reported that mindfulness improved students’ behavior and ability to focus, as well as teachers’ sense of well-being—though the research design had several main points of weakness, mostly involving the challenge of measuring children directly rather than through teacher assessments. Mindful Schools’ new research director told me that the group is eager to conduct more studies that are even better-planned, focusing more on the efficacy of the kind of integrated training Gonzalez received. The field is so new that techniques evolve rapidly, constantly going through phases of trial-and-error—so it remains to be seen whether current or future findings can convince skeptics of mindfulness’s effectiveness.

* * *

Back in the Bronx, after a minute or two of the day’s mindfulness exercise, his own eyes also closed, Gonzalez ran through a list of emotions: Happy. Sad. Excited. Mad. Bored. Loving. Worried. Jealous. Silly. The second item on this list seemed to especially resonate with an 18-year-old at the front of the classroom, a young woman with dark skin, shimmering pink lip gloss, and perfectly plucked eyebrows. Sitting up straight with her hands in her lap, her composed posture belied the challenges she faced shortly before transferring to Arturo A. Schomburg two years earlier.

“I didn’t know anybody. I was very depressed. I didn’t want to be in school,” she told me in a hushed voice at the end of class. Shortly before transferring to this school, her favorite big brother had been hit by a car. She said she’d watched him fall into a coma, and sat by his side until his heart stopped; soon after that, she’d seen one of her friends get shot in the head and bleed to death in the street. During the quiet minutes set aside for mindfulness exercises in class, she would often cry.

Now, she writes in perfect, neat script as she fills out a worksheet to accompany the day’s mindfulness exercise. But she told me she wasn’t always so eager to participate. “I used to write, ‘I hate this, I don’t want to do this.’ I ripped those papers up,” she said. But one day when she was in a particularly dark mood, something clicked. “Argos told me to close my eyes. Then he said, ‘Connect to your breath.’ He always used to say it, but I never really did it until then.” Gonzalez told me that his Mindful Schools training had specific segments dedicated to working with trauma.

“I noticed that I could feel [my breath] in my chest,” she told me, “And at that moment, I felt so relieved. The only thing I could think in my mind was, ‘I’m ok.’ And, I don’t know—from that day on, it just didn’t hurt anymore.” She told me she hadn’t been in fights the way she once used to. Her four other brothers are in jail, and she is convinced it’s because they didn’t get the mindfulness training she now has. “Your emotions drive you mad,” she said, but escaping them is possible by “focusing on now.” (Our conversation also benefitted from the fact that I myself have some knowledge of mindfulness; I discovered it during a year off from college as I struggled with anxiety and depression.)

Another student told me she was skeptical about mindfulness but admitted that it could be helpful. She told me that she initially refused to do the exercises, sitting defiantly while others participated. Some of the tasks—like tapping your thumb to each finger individually, to narrowly focus attention on your fingertips—did nothing but irritate her. Eventually, though, she realized she was alone in her resistance, and she began to go through the motions, largely because she likes and respects Gonzalez. She was also struck by a movie Gonzalez showed them that compared two jails, one that trained prisoners in mindfulness and one that didn’t. The prisoners who learned mindfulness were much happier and more successful when they got out. Still, ultimately, she maintains that she doesn’t see the point.

* * *

Beyond the issue of scientific evidence, bringing mindfulness into classrooms raises other questions: How does it fit into the traditional teaching model? Could any teacher teach mindfulness, or does it require a significant personal investment? Is opening teachers up to dealing with their students’ emotional and psychological needs, in addition to their academic ones, encouraging a blur between teacher and therapist?

Gonzalez doesn’t think so. “My intention as a mindfulness instructor is to give students some very simple and basic tools so they can learn to self regulate. That’s the beginning and end of it.” When a student is dealing with emotional trauma, Gonzalez said he’s been taught to keep his advice general—to remind the student that everyone suffers and feels pain, but that life is a gift to be treasured.

The clinical social worker at Gonzalez’s school—a large man with a warm baritone voice—thinks mindfulness supports the school’s overall SEL mission. “At times all the roles blur—teachers, therapists, social workers. Especially in a school like this. If you don’t address the noise in a kid’s head that they bring in from the outside, I don’t care how good a teacher you are, you’re not going to have much success.”

He was convinced that Gonzalez is on the right track; and that all teachers should get something akin to mindfulness training, given that they must deal with undiagnosed mental conditions on a regular basis. While they are not therapists, they “can at least ease some of the stress in the moment. Long enough to have somebody intervene.”

Greenberg’s view about the teacher-as-therapist issue is also clear: “Teachers teach many things that are therapeutic. They are managing children’s behavior all day long, but that doesn’t make them therapists, that makes them good teachers. Some of the same ideas we teach in therapy are also applicable to all people.”

Beyond helping his students, Gonzalez also thinks mindfulness helps him to cope with the strains of teaching. He believes he now draws clearer lines in his relationships with students—giving them the skills to help themselves, rather than feeling that he needs to be the one to heal them—and copes more healthily with the trauma the job exposes him to, whether directly (in a previous teaching job, he said a student once stumbled into his office bleeding from a stab wound) or indirectly through working with a grieving student.

Gonzalez ultimately thinks that mindfulness may go furthest if applied to teacher education as a way to help prevent burnout—a major issue, given that 20 percent of teachers in high-poverty schools leave within their first year. Greenberg agrees. One of the ongoing research projects he and his colleauges are involved in is the Cultivating Awareness and Resilience in Education (CARE) program, funded by the U.S. Department of Education, that focuses on the well-being of the teacher and instructs teachers on emotional awareness, techniques for emotion regulation, and ways to apply these skills to teaching. Greenberg and others suspect that mindfulness specifically tailored to teachers and their struggles—stress and time management, for example—and incorporated into their initial training might do as much or more to improve classroom performance than trying to teach children directly. In its annual surveys, Mindful Schools has found that a majority of the teachers it has trained experienced lowered stress, more connection with students, and higher job satisfaction.

By the end of Gonzalez’s morning class, the quiet, focused tone had long faded. Several students stood up from their desks, leaning over each other, laughing, knowing that personal challenges await them outside the classroom, just as they always have.

Over the chatter, the student with the glossy pink lips told me that hearing Gonzalez say the word “sad” triggered a flashback to all of those overwhelming memories of grief and pain she has been working to move beyond, but that it was okay. “Those feelings are there, but they won’t kill me,” she said. “I still have my days where it’s not easy, but mindfulness helps me a whole lot. Honestly, I feel like if I’d had this before, it would have been easier.”

Lauren Cassani Davis is a former editorial fellow at The Atlantic.