Every year that Hiroo Ishihara's children live in Washington they fall further behind. They read and write less Japanese, they study the history of the United States insted of Japan and they learn less math than if they were in school back home.

But when Ishihara's two-year assignment at the National Institute of Health ends this summer and his children -- Akiko, 11, Naoko, 9, and Seiko, 6 -- reenter the highly structured Japanese schools, the transition, while difficult, will be eased by the Saturday mornings they spent in a rented building in Rockville.

There for six hours every week the Ishihara children join 300 other sons and daughters of Washington's Japanese community for intensive classes in Japanese, math and social studies. It is called, appropriately, the Japanese School, and it was set up in 1964 with the aid of the Tokyo government to offset the reentry problems children usually experience after they have spent time in an English-speaking society.

"It's the only way to make it possible for them to return to Japanese schools," says Ishihara.

Naoko Ishihara, for example, must know about 400 of the 1,850 basic Japanese characters by the end of this year, and her sister Akiko must have mastered a total of 900 by the time she finishes the sixth grade.

The Japanese population in Washington, consisting of about 30,000 diplomats, businessmen and their dependents, is not large enough to support a full-time school, so the children receive an American education during the week and study at the Japanese school on Saturdays, in a building rented from the Georgetown Preparatory School.

The Japanese government sends textbooks free of charge for grades one through nine, the compulsory education grades, and pays the salary of the principal, who is assigned to the post by the Ministry of Education. The 16 teachers are accredited instructors, many of them wives of diplomats or bankers.

Shuzo Nagata, an employe of the World Bank whose 12-year-old son Shu attends the school, has lived abroad before and has seen the effects on his daughter.

"After five years in England, when she returned to Japan and entered primary school, other people thought her a kind of foreigner, regardless of what she looks like," he said. "If the youngsters cannot speak Japanese perfectly, they will have problems."

Eleven-year-old Mika Hamada said she doesn't mind going to school on Saturday, sitting through three classes each an hour and 40 minutes long.

If the work is hard, the atmosphere is hardly grim. The hallways ring with merry shouts from the usually quiet, well-disciplined youngsters, and techers admit to being more lax with their behavior than would be normal in Japan.

"A lot of them have language trouble in American schools, so it is a kind of catharsis for them to come to this school," said Hiroshi Suzuki, a social studies teachers.

After one to three years overseas, almost all of the students at the school return to Japan. Satoshi Fujimoto, the 16-year-old son of a Japanese television correspondent, is an exception. He asked to stay behind when his parents returned to Japan two years ago. He's now a boarding student at Georgetown's Prepatory School.

His decision means he will complete his high school and college education here.

"I went back to Japan in the summer of 1978 and saw the educational system there. It was really the pits," he said. "It's like the rumors I've heard. You take tests and try to get into better schools. It's like you're studying for the next school and not yourself.

"Besides, it's a pain getting into Japanese schools after five or six years here."

But he comes to school on Saturdays. "I acknowledge myself as a Japanese and I don't want to forget the language."