WASHINGTON - American teenagers have less mastery of science and mathematics than peers in many industrialized nations, according to scores on a major international exam released yesterday.

Education experts say results of the 2006 Program for International Student Assessment highlight the need for changes in classrooms and in the No Child Left Behind law. The average science score of U.S. 15-year-olds lagged behind that of students in 16 of 30 countries in the Organization for Economic Cooperation and Development.

In math, U.S. students trailed counterparts in 23 countries.

"How are our children going to be able to compete with the children of the world? The answer is not well," said former Colorado Gov. Roy Romer, chairman of Strong American Schools, a nonpartisan group seeking to make education prominent in the 2008 presidential election.

The PISA test, given every three years, measures the ability of 15-year-olds to answer math and science problems. About 400,000 students, including 5,600 in the United States, took the 2006 exam. There is also a reading portion, but the U.S. results were thrown out because the tests were printed incorrectly.

Students in Finland earned top scores in science and math. Mexico was at the bottom of the pack.

The results underscore concern in some quarters that too few U.S. students are prepared to become engineers, scientists and physicians and that the nation may lose ground to economic competitors. An expert panel appointed last year by President Bush is preparing to recommend ways to improve public school math instruction, with a focus on algebra.

On the science portion of the 2006 exam, U.S. students, mostly 10th graders, earned an average score of 489 on a 1,000-point scale, 11 points below the average of the 30 countries. Canada, Japan and South Korea were among countries whose students outperformed their U.S. counterparts.

In math, only four countries had average scores lower than the United States.