IT'S good for public-school students in the United States to learn about the diversity of cultures that have contributed to American society.An appreciation of that rich cultural mix can bring needed excitement to what has too often been a yawn-inspiring social studies curriculum. It can also help instill such essential social attitudes as tolerance. But the questions raised by critics of some efforts to implement multicultural curricula have to be honestly faced. When does an emphasis on ethnic identity start to undermine social cohesion? Are attempts to focus on the accomplishments of nonwhite peoples sometimes veering away from the facts of history? In his dissent from the report of New York's state-appointed committee to review the social studies curriculum, historian Arthur Schlesinger Jr. noted a lack of stress on those elements that bind the nation together. He wrote, "The bonds of national cohesion in the republic are sufficiently fragile already. Public education should aim to strengthen, not weaken, them." A fundamental bond of cohesion is the set of political ideals - such as free expression, equal opportunity, and due process of law - that are embodied in the US Constitution. These can be approached with an emphasis on the nation's failure to live up to them in its treatment of blacks, Indians, Japanese-Americans, Hispanics, and others. Or the emphasis can be placed on the enduring nature of these ideals and the slow but constant effort to attain them. That decision, largely, is the teacher's. Children will be best served by an honest assessment of their country's history - baring the tragic shortcomings, but recognizing the achievements and the possibility of greater things to come. Above all, youngsters have to be shown that concepts like democracy, freedom, and equality are not the possessions of any group. They're universal. White men of European background may have given them clear expression 200 years ago, but Americans of all ethnic backgrounds and both genders have made these concepts their own, clarifying and furthering them. It's less important that students be taught to say "native American" instead of "Indian," or "enslaved person" instead of "slave," than it is that they grasp something of the human circumstances endured by both peoples. A citizenry aware of past injustices is less likely to tolerate new ones. Current experiments in multicultural education, like the Africa-centered curriculum being tried in Atlanta and other cities, are showing some positive results: lower truancy rates and improved student performance. But a dogmatic emphasis on semantics, or distortions of history to serve philosophical ends, could backfire. As other generations of American students know, disillusionment can set in when you learn that the course of history was not quite that portrayed back in high-school social studies.