JAPAN'S education system has come under sharp criticism in the past decade, despite a record of successfully producing literate and able workers for industry.Some new social problems, such as a falling birth rate, are now blamed on the rigidity and excessive rigor of the system. The main target of criticism has been the Ministry of Education's bureaucrats, who hold central control over textbooks, teachers, and the university entrance exams that dominate the upbringing of so many Japanese children. Schools must maintain strict rules, some even specifying the color of underwear students must wear with uniforms. Proposals for reform, many of them made in 1987, were aimed at helping the Japanese to cope better with the outside world and to lessen the pressure on families. But few have been implemented. Moral education is being introduced. And students returning after being overseas with their parents are now better treated than in the past, when they were largely stigmatized. The latest idea is to drop Saturday classes. But opponents say that parents will just make their children go to cram schools on Saturday instead. Another proposal, based on concerns over a shortage of engineers and scientists, is to have special classes for students considered smart in math and science. But this would break the equality of education that was designed to prevent a return to the elitism of prewar schools.