IF THE JUSTIFICATION exists in social theory or constitutional law for busing a black child miles away from his racially integrated neighborhood public school to attend a virtually all-black schoo somewhere else, we haven't heard it. Well, let us amend that remark: We haven't heard it since the long-gone defenders of pre-1954-style formal racial segregation in the schools sought to explain how it was that they were transpoting black children every which way and as far as it was necessary to go to make sure that they didn't sit in classrooms with white children. But it was not in the name of segregating schools, but rather of desegregating them that the practice began to recur recently in Prince George's County. And that -- along with a number of other grotesque effects -- was what prompted the Prince George's school board chairman, Norman H. Saunders, and the president of the local NAACP, William Martin, to agree to a halt to certain kinds of busing. If their understanding, as it is called, holds, it will represent one of the rare and notable moments in our modern public life when good sense prevailed over rigid dogma.

That will strike you as a pretty cynical observation. But only consider how our contemporary landscape is strewn with the wreckage of social policies and programs that were generous in theory and even right for a particular problem at a particular time, but which 1) became cruel or stupid in their impact as conditions changed, and yet 2) could not, for reasons of political face-saving or bureaucratic lethargy, be changed themselves. What has changed in Prince George's since the desegregation plan went into effect in January 1973 is not just the attitude of many of the county's once combative residents, black and white, but also the demography of the area. The black population has gone up dramatically in many formerly all-white neighborhoods. There has been a considerable degree of "white flight." One result is the busing of black kids from now-intergrated neighborhoods to now-segregated schools. This is part of the new reality to which people like Mr. Martin and Mr. Saunders are seeking to adjust the county's school-integration program.

It should surprise no one that there already is and no doubt will continue to be deeply felt and very voluble resistance to any alteration along the lines of this new understanding. Prince George's came to its desegregation plan by about as prolonged and agonizing a process as could be imagined, moving from legally enforced pre-1954 segregation to administratively sustained segregation to tradition-sanctioned segregation. Only with great anguish and acrimony and after battles that are still vivid for many within the community did the school board and the county's political leadership finally yield to the inevitable federal intervention. Some people, among them many of Mr. Martin's NAACP colleagues, who fought that fight and who know how hard-won was the desegregation of Prince George's schools, will be inclined to yield nothing, change nothing. And the same will be true of many of those who resisted and who fear that any alteration of the plan will only open the way to new and disruptive suits.

But our sense of it is that resisters, black and white, will be making a big mistake. In fact, Prince George's protracted misery on this subject may be said to have proceeded in large part from the inability of its leaders to move beyond the rigidities of obsolete doctrine and overturned laws or to respond to dramatically changed social, racial and political conditions. And in that sense they may be said to have inadvertently invited the harsh federal court order and the upheaval that followed. It was a lesson in lost opportunities: The community passed up a chance to design its own desegregation plan. The county wouldn't act, so the courts and HEW did. Now Prince George's is lucky enough to have leaders in positions of authority who want to act in the opposite way, to respond in a timely and sensible fashion to the changes that are apparent for all to see. It will be a great pity and a great waste if other voices in the community reject this wise counsel.