Commentaries on the 50th anniversary of the Supreme Court's 1954 decision in Brown v. Board of Education have ranged from celebration of its bold attack on segregation to bitter complaints about its failure to bring an end to racial isolation in the nation's schools. Before deciding how to judge the half-century since Brown, I would suggest we examine how much interracial contact in the nation's schools has actually changed.

To conclude that nothing has in fact changed would be wildly inaccurate. The differences in interracial contact between then and now are little less than revolutionary. In 1954, four out of every 10 students in the United States attended schools that were segregated by law. Today the region that accounted for the bulk of those states, the South, has the least segregated schools in the country. Black and white students march in bands, sit in class and compete on teams together to an extent unimagined in the pre-1954 South. Nationwide, on average, the school a white student attends is now one-quarter minority. The black student is in a school where, on average, half the students are white. Detroit -- the metropolitan area with the most segregated schools in 2000 -- would have ranked only 35th in measured segregation in 1970, among 332 metropolitan areas.

But as with a child running up a descending escalator, the changes wrought by Brown faced countervailing forces, the most important being white reluctance to embrace racially mixed schools. "White flight" to the suburbs led to the growth of predominantly white school districts there, reducing by a quarter the advances made toward desegregation between 1970 and 2000. These city-suburban disparities were most severe in the North and Midwest, where school districts tend to be small, numerous and homogeneous. Whites also invested in private schools, although these were not a major avenue of escape except in the handful of predominantly black nonmetropolitan districts in the South where whites virtually abandoned the public schools in the wake of desegregation orders.

Now, having been freed of the oversight of federal courts, some school districts in the South are gradually returning to assignment policies based on neighborhood schools, with the predictable result of increasing the number of racially isolated schools.

Consider Charlotte-Mecklenburg, a North Carolina district transformed by the Supreme Court's landmark 1971 Swann decision, which endorsed cross-town busing as a means of achieving racial balance. Following a ruling by a lower federal court in 2001 that the district had erased the vestiges of past segregation, the Charlotte school board adopted a new pupil assignment plan based on parental choice. This plan allowed more students to go to neighborhood schools, with the result that 23 percent of black students now attend schools with 90 percent or more minority enrollment.

Clearly, the trend in the United States in recent years is toward a slow but marked reversal of Brown. But does it matter if racially isolated schools are reappearing in American communities?

There is good reason to think that it does. As military and corporate leaders argued in briefs submitted to the Supreme Court last year in connection with the affirmative action challenges brought against the University of Michigan, the racial and ethnic composition of our country makes diversity a necessary ingredient in a good education. In his dissenting opinion in another Michigan case in 1974, Thurgood Marshall put it simply: "Unless our children begin to learn together, there is little hope that our people will ever learn to live together."

The federal courts are taking an increasingly laissez-faire approach to the question of racial desegregation in our schools. If we do want our children to learn together, perhaps for no better reason than, as Marshall suggested, to ensure that they can live together, local school boards will have to accept the mantle of preserving the fragile gains achieved in the post-Brown era. Some districts have demonstrated that they can meet the challenge. They have fostered interracial contact through such policies as attractive magnet-school offerings, school assignments that limit the proportion of low-income students in any school and voluntary transfers across district lines to enhance racial diversity. But these policies will not be easy to sustain without an understanding of and commitment to racial diversity.

The writer is a professor of public policy studies, economics and law at Duke University and the author of "After Brown: The Rise and Retreat of School Desegregation."