American high school seniors ranked close to last among their international peers in a new math and science test, according to scores released yesterday. Their showing was much worse than the marks that American elementary and middle school students have earned on similar international exams in the past two years.

• Among the 21 countries participating in the Third International Science and Mathematics Study, American students only outperformed students from Cyprus and South Africa.

• Their scores were comparable to those of students from Russia, Italy and the Czech Republic.

• Countries that excelled on the exam included the Netherlands, Sweden and Norway -- but Canada and New Zealand also registered higher marks than the U.S.

• Students from the 21 countries averaged 2.6 hours daily studying homework -- while U.S. students averaged only 1.7 hours.

Experts say that one reason for the relatively poor performance of U.S. high schools seniors is that they take fewer math and science courses than their peers in other countries.

• While 66 percent of U.S. seniors sign up for math courses, the average among all participants was 79 percent.

• In the U.S., 53 percent take science -- compared to 67 percent overall.

Fifty-five percent of U.S. seniors work at a paid job three or more hours daily -- more than three times the 18 percent average for all students participating. But 73 percent of U.S. students use a computer, compared to the overall average of 57 percent.

In the U.S., the test was administered to about 10,000 seniors selected randomly from more than 200 public and private high schools. It was supervised by the U.S. Department of Education.