U.S. students once again lag behind many of their Asian and European peers on a global exam, a trend often blamed on child poverty and a diverse population in U.S. schools.

Education Secretary Arne Duncan called the results a "picture of educational stagnation" as U.S. students showed little improvement over three years, failing to score in the top 20 on math, reading, or science.

Students in Shanghai had the top scores in all subjects, and Singapore, South Korea, Japan, and Hong Kong students weren't far behind on the PISA, given to 15-year-olds. In math, 28 nations fared better than the United States; in science, 22 scored higher.

"Americans have got a thousand reasons that one country after another is surpassing our achievement, and I have yet to find a good excuse," said Marc Tucker, president of the National Center on Education and the Economy. - AP