Even America's top high school seniors - those taking the toughest math and science courses - performed far worse on an international test than similar students in most other countries.

"This is unacceptable," Education Secretary Richard Riley said Tuesday of America's showing among the lowest of 21 nations on the Third International Mathematics and Science Study.Not only did typical American seniors do poorly, but those who took physics and advanced math courses such as calculus and analytical geometry also performed at or near the bottom when compared with students in other countries who took similar courses.

"Our best students in mathematics and science are simply not `world class,' " said William H. Schmidt, a Michigan State University professor and national research coordinator for the federally supported project. "Even the very small percentage of students taking Advanced Placement courses are not among the world's best."

Riley and others blamed a dearth of qualified teachers and easy graduation requirements. About half of physics teachers lack a major or minor in that subject. Likewise, about half of college-bound seniors have not taken four years of science.

Others, like Schmidt, said that students in other countries begin learning elements of algebra, geometry, physics and chemistry in middle school while science and math teaching in this country often is repetitive and unchallenging. Previously reported results for fourth- and eighth-graders found the relative U.S. performance was stronger in those grades.

Teaching here usually is done layer-cake fashion, devoted to one subject per year, while the curriculum elsewhere blends disciplines, he says.

Some American high schoolers agreed that their preparation could be better.

"It could have been better if in middle school or in elementary school I got the training I was supposed to be getting," said Kamille Brown, 17, a senior in Miami. "It's important to start with the youth and have them get the training that they're supposed to be getting at the elementary and middle-school level so that they can be better established."

Kristin Schulte, a visiting 16-year-old junior from Wichita, Kan., said that she had switched from a Catholic school to a public school in the ninth grade, and repeated much of what she had learned in the eighth.

"Biology is like a basic review of seventh- or eighth-grade stuff," she said. "I should have gone to something harder."

She plans to take physics as a senior after her PSAT scores showed the price for not taking chemistry or physics.

The international tests, given in the 1994-1995 school year, measured general math and science knowledge, defined as the knowledge needed "to function effectively in society."

In general math knowledge, the U.S. students were below the international average and closest in performance to students in the Czech Republic, Italy, Lithuania and the Russian Federation. Americans scored significantly lower than students in 14 other countries, mostly European, but including Australia and Canada. Asian countries did not take part in the 12th-grade study.

In general science, U.S. students also scored below the international average alongside the Czech Republic, France, Germany, Hungary, Italy, Lithuania and the Russian Federation. Eleven countries scored significantly higher.

Only 42 percent of U.S. students could answer why spike heels might cause more damage to floors than might ordinary heels. Sixty percent of international students knew the answer - that more energy is concentrated in a smaller area.

American students who took advanced mathematics scored far below the international average, along with students from Austria, the Czech Republic, Germany and Italy. Eleven countries, including the Russian Federation, did much better.

In physics, the United States ranked at the bottom with Austria. Fourteen other countries had scores that were significantly higher.