WASHINGTON (AP) -- Teens from Asian nations dominated a global exam given to 15-year-olds, while U.S. students showed little improvement and failed to reach the top 20 in math, science or reading, according to test results released Tuesday.

American students scored below the international average in math and about average in science and reading.

The top average scores in each subject came from Shanghai, China's largest city with more than 20 million people. Singapore, South Korea, Japan, and Hong Kong were among the participants with students scoring at the top on average in each subject. Vietnam, which had its students participate for the first time, had a higher average score in math and science than the United States.

Education Secretary Arne Duncan called the results a "picture of educational stagnation."

"We must invest in early education, raise academic standards, make college affordable, and do more to recruit and retain top-notch educators," Duncan said.

About half a million students in 65 nations and educational systems took part in the 2012 Program for International Student Assessment, or PISA, which is coordinated by the Paris-based Organization for Economic Cooperation and Development, or OECD.

Most results come from a sampling of scores from countries as a whole, but in China it was given in select regions. The Education Department's National Center for Education Statistics released the results.

The test, which is given every three years to 15-year-olds, is designed to assess how students use what they've learned inside and outside of school to solve problems.

U.S. scores on the PISA haven't changed much since testing started in 2000, even as students in countries such as Ireland and Poland have shown improvement and have surpassed U.S. students.

"It's hard to get excited about standing still while others around you are improving, so I don't want to be too positive," said Jack Buckley, commissioner of the National Center for Education Statistics. But he added that the country has a respectable foundation on which to build.

American students historically have not had high marks on international tests. Factors often cited include high rates of child poverty and population diversity.

In contrast to the PISA results, American fourth- and eighth-graders over time have made some progress in reading and math on an assessment referred to as the Nation's Report Card, even though recent results found the vast majority of the students still are not demonstrating solid academic performance in either subject.

In the education community, Finland has drawn notice for its past test performance, but this year its average PISA score dropped in all three subjects, most pronounced in math. Finland's students did better on average than those from the United States.

Shanghai students also topped the PISA test in 2009. Tom Loveless, a senior fellow at the Brookings Institution, said the school system in Shanghai is not equitable and the students tested are children of the elite because they are the ones allowed to attend municipal schools because of restrictions such as those that keep many migrant children out.

"The Shanghai scores frankly to me are difficult to interpret," Loveless said. "They are almost meaningless."

Buckley said U.S. officials have not seen any evidence of a "biased sample" of students tested in Shanghai. If the entire nation was included, he said it's unclear what the results would show.

Overall, NEA President Dennis Van Roekel said among the Asian nations dominating the test, "The one thing they all have in common is that they make a real commitment to education for all kids and nothing deters them from that vision, and then they do what's necessary to make that happen. In the United States, we don't have the commitment for all kids and it needs to change."

One indicator of performance is how many students scored at a high level on each subject tested. In the United States, 9 percent of test-takers hit that mark in math, 7 percent did so in science and 8 percent did in reading. Fewer U.S. test-takers hit that mark in math than the international average. However, they performed at about the international average in the other two subjects.

Students from all states were tested. But for the first time, three states, Massachusetts, Connecticut and Florida, opted to increase participation in PISA to get more state specific results.

Average scores from Massachusetts were above the international average in all three subject areas. Connecticut's scored on average around the international average in math and higher than the international average in science and reading. Florida students on average scored below the international average in math and science and around the international average in reading.

The test is based on a 1,000-point scale. Among the findings:

-- In math, the U.S. average score was 481. Average scores ranged from 368 in Peru to 613 in Shanghai. The international average was 494.

-- In science, the U.S. average score was 497. Average scores ranged from 373 in Peru to 580 in Shanghai. The international average was 501.

-- In reading, the U.S. average score was 498. Average scores ranged from 384 in Peru to 570 in Shanghai. The international average was 496.

Some schools and districts are opting to take a "mini-PISA" called the OECD Test for Schools designed to allow a school to see how it compares internationally. About 100 schools participated last year in a pilot program pushed by the nonprofit America Achieves and the OECD, and more are expected to do so this school year. Participating schools receive an in-depth analysis of the test results along with results of a survey given to the students at testing time. The schools and districts involved decide whether to publish results.

Jon Schnur, executive chairman of America Achieves, called the test an "opt-in empowerment tool."

"Some say, well our education has failed in the U.S. Well, that's not true. Some say education is fine in the U.S. That's not true either," Schnur said. "We need to do a better job helping our students keep pace with the changed world."

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