Judy Mann {"Give Longer School Days a Chance in Fairfax," Metro, Nov. 14} wrote, ''The Japanese school year, for example, is 243 days. The United States school year averages 180 days. U.S. youngsters are spending two full months less time in school than Japanese children.'' In fact, at 20 working days per month, it would take more than an additional three months to approach the Japanese.

However, even this dramatically understates the relative quality of education American schoolchildren receive, because Japan's teaching jobs are far more difficult to get, their schools are highly attractive to top candidates and about four-fifths of Japanese students receive private tutoring classes.

The Japanese do about four times more homework than U.S. students, including during the summer, and also have longer school days; (in 1988, William Bennett noted our average six-hour day compares unfavorably with the eight-hour day common in Western societies). Most disturbing of all is that U.S. kids watch 25 hours of TV a week, doing three to five hours of homework a week. It is not surprising that our children finish at the bottom of every major index of learning.

However, many U.S. kids don't even get this substandard level of education. The U.S. high school dropout rate is 29 percent, compared with a 12 percent rate in Japan, according to 1984 U.S. Department of Education statistics. The dropout rate in American cities is even higher. These kids have been made outsiders in our society by a failed system.

The U.S. problem is made worse by the use of local funding for U.S. education, which makes the less wealthy communities receive even poorer education, while many nations, including Japan, fund education more centrally with a decidedly egalitarian and more consistent result. The inequality of U.S. education makes our future work force less adequate still.

These children are the workers of tomorrow in the international economy. Their education will determine the quality of their jobs, job security in their careers and the future soundness of the economy on which today's parents will depend for all medical and retirement benefits. While we might choose not to hold American students to the high standards of our competitors, clearly we must do a better job by our children's future than these statistic show.

THOMAS P. O'BRIEN Director for Research Horizon Institute for Policy Solutions Charlottesville