NEW YORK (AP) _ A new study concludes that American pupils are among the weakest mathematics performers among industrialized nations and blames curricula which lack ''challenge and focus.''

The report, ''The Underachieving Curriculum: Assessing U.S. School Mathematics from an International Perspective,'' pointedly disputes some of the most often-heard explanations for the weak showing, such as poor teacher preparation, not enough class time devoted to math, and over-large class sizes.

Instead, the latest report, released Sunday, puts primary blame on the curriculum which, in typical U.S. classrooms, is pointlessly repetitious.

''The U.S. curriculum from an international point of view lacks challenge and focus. The curriculum typically keeps revisiting concepts, but the trouble is, with each revisit, we're not adding anything new,'' said Kenneth J. Travers, a University of Illinois mathematics education professor who was one of seven co-authors of the report.

''One would have expected more from the advanced industrialized country that has provided the world with so much technical leadership,'' Travers said.

''In school mathematics the United States is an underachieving nation and our curriculum is helping to create a nation of underachievers,'' said the study.

The report will be a main topic at an international math symposium at the National Academy of Sciences in Washington Jan. 15-16, and comes on the heels of a study released by U.S. Education Secretary William J. Bennett extolling Japanese elementary and secondary schools. He suggested that Americans should borrow some educational lessons from Japan, especially the view that ''progress can be made by practically anyone who tries hard enough.''

Challenging the idea that large class size is responsible for poor U.S. math performance, the report pointed out that in Japan, average eighth-grade class size is 40, and in 12th grade it is 43. Both exceed U.S. averages of 26 students in eighth grade, and 20 in 12th grade.

And the average amount of time devoted to math in U.S. schools was 144 hours per year, compared to 101 hours for Japanese youngsters, according to the study.

The report also questions the common practice of tracking students early in their school careers according to their mathematics ability.

The problem, Travers said in a telephone interview, is that typically the eighth grade math curriculum in U.S. schools repeats elementary math, while students in other countries move along briskly to new concepts.

''In the eighth grade we just do sorting and tracking. Significant portions of kids are kept in grade school arithmetic. That's not found to nearly that extent in other countries, and the Japanese don't track students at all,'' Travers said.

Among other recommendations, the report called for renewed scrutiny of math textbook quality, increased status and rewards for math teachers, and improved professional development programs for teachers.

The study draws heavily from the widely publicized ''Second International Mathematics Study'' released in 1984 showing U.S. eight- and 12th graders ranking no better than average, and often far worse, in a variety of mathematical subjects compared with 19 other nations and territories.

That study of 12,000 pupils in those 20 countries found, for example, that Japanese eighth-graders had the best achievement scores in arithmetic, algebra, geometry, statistics and measurements, the five topics studied.

The best showing by U.S. eighth-graders was in arithmetic, where they ranked 10th among the 20 countries studied. But they ranked 12th in algebra, and 16th in geometry.

Among 12th graders in college preparatory high school programs, Hong Kong students ranked first among 15 nations studied in each of six mathematical areas evaluated, with Japan a close second.

But American 12th graders were second from the bottom in advanced algebra, besting only Thailand students. And they ranked 12th out of 15 nations in both calculus and geometry.

The countries and territories studied besides the United States were Belgium (French and Flemish), British Columbia, England/Wales, Finland, France, Hong Kong, Hungary, Israel, Japan, Luxembourg, the Netherlands, New Zealand, Nigeria, Ontario, Scotland, Swaziland, Sweden, and Thailand.