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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 decadeslong 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 (PISA), were announced 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.

Overall, 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.

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 — 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.

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, Koretz said, states and districts did not always effectively carry out the Common Core or other initiatives.

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.

The top performers in reading were four provinces of China. TheU.K., Japan and Australia performed similarly to the U.S.

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