The performance of American teenagers in reading and math has been stagnant since 2000, according to the latest results of a rigorous international exam, despite a decades-long effort to raise standards and help students compete with peers across the globe.

And the achievement gap in reading between high and low performers is widening. Although the top quarter of American students have improved their performance on the exam since 2012, the bottom 10th percentile lost ground, according to an analysis by the National Center for Education Statistics, a federal agency.

The disappointing results from the exam, the Program for International Student Assessment, were announced on Tuesday and follow those from the National Assessment of Educational Progress, an American test that recently showed that two-thirds of children were not proficient readers.

Over all, American 15-year-olds who took the PISA test scored slightly above students from peer nations in reading but below the middle of the pack in math.

[Readers weigh in on American students’ PISA test scores.]

Low-performing students have been the focus of decades of bipartisan education overhaul efforts, costing many billions of dollars, that have resulted in a string of national programs — No Child Left Behind, Race to the Top, the Common Core State Standards, the Every Student Succeeds Act — but uneven results.

There is no consensus on why the performance of struggling students is declining. Education experts argue vociferously about a range of potential causes, including school segregation, limited school choice, funding inequities, family poverty, too much focus on test prep and a dearth of instruction in basic skills like phonics.

About a fifth of American 15-year-olds scored so low on the PISA test that it appeared they had not mastered reading skills expected of a 10-year-old, according to Andreas Schleicher, director of education and skills at the Organization for Economic Cooperation and Development, which administers the exam.

Those students, he said, face “pretty grim prospects” on the job market.

Daniel Koretz, an expert on testing and a professor at the Harvard Graduate School of Education, said recent test results showed that “it’s really time to rethink the entire drift of policy reform because it just isn’t working.”

Because the United States lacks a centralized system for teacher training or distributing quality instructional materials to schools, Professor Koretz said, states and districts did not always effectively carry out the Common Core or other initiatives.

The Common Core, which began almost a decade ago, has been a national effort by governors, state education chiefs, philanthropists and school reformers to enrich the American curriculum and help students compete with children around the world. Its priorities include increasing the amount of nonfiction reading, writing persuasive essays using evidence drawn from texts and adding conceptual depth in math.

The effort became a political lightning rod, with the left opposing a new generation of standardized tests tied to the Core, and the right seeing the effort as an unwelcome intrusion into local control of schools. Some states that initially signed on to the Core later rejected it.

Even in those places that stuck with the effort, the curricular changes that flowed from the Common Core could be made without necessarily improving the quality of teaching, Professor Koretz said.

He suggested a renewed focus on classroom instruction, and on providing students and families who are poor, or are recent immigrants, with support like social workers and translators.

The most recent PISA test was given in 2018 to 600,000 15-year-olds in 79 education systems around the world, and included both public and private school students. In the United States, a demographically representative sample of 4,800 students from 215 schools took the test, which is given every three years.

Although math and science were also tested, about half of the questions were devoted to reading, the focus of the 2018 exam. Students were asked to determine when written evidence supported a particular claim and to distinguish between fact and opinion, among other tasks.

The top performers in reading were four provinces of China — Beijing, Shanghai, Jiangsu and Zhejiang. Also outperforming the United States were Singapore, Macau, Hong Kong, Estonia, Canada, Finland and Ireland. The United Kingdom, Japan and Australia performed similarly to the United States.

Among the countries that demonstrated improvement on the test were Portugal, Peru and Colombia.

There were some bright spots for the United States: Achievement gaps between native-born and immigrant students were smaller than such gaps in peer nations.

Mr. Schleicher, of the Organization for Economic Cooperation and Development, said it was a common misconception that socioeconomic achievement gaps in the United States were much larger than those in the rest of the world. Three percent of American children from poor families were top performers in reading, compared with an average of 4 percent of poor children among O.E.C.D. countries.

In math, socioeconomic status explained 16 percent of the variation in American performance, similar to the average of 14 percent across O.E.C.D. nations.

Mr. Schleicher said that differences in school quality affected the performance of American students less than it affected the performance of students in many other nations — meaning that in the United States, there is more achievement diversity within schools than across schools.

Some education leaders said they saw no reason to drastically change policy directions.

William G. McCallum, a mathematician and one of the lead writers of the Common Core State Standards, said he remained hopeful that a strategy of rigorous standards, quality classroom materials and effective teacher training could improve student achievement.

He noted that Washington, D.C., which has been committed to the Common Core, had recently demonstrated impressive performance gains.

“Frustration is understandable,” he said of low test results. But, he added, “Maybe this is just a really hard problem.”