Washington, D.C. ? American teenagers have less mastery of science and mathematics than peers in many industrialized nations, according to scores on a major international exam released Tuesday.

Education experts say results of the 2006 Program for International Student Assessment highlight the need for changes in classrooms and in the federal No Child Left Behind law. The average science score of U.S. 15-year-olds lagged that of students in 16 of 30 countries in the Organization for Economic Cooperation and Development, a Paris-based group that represents the world’s richest countries. U.S. students were further behind in math, trailing counterparts in 23 countries.

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.

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