In some areas, because of their location schools may be generally one race of the other. If other races are not found in a certain housing area then only those that live in that area will be able to attend the school. I think that if an area wants diversity then they will have to participate in transporting other racing in to the school.
People tend to live in segregated neighborhoods for a reason. If people wanted their kids to be desegregated, they would move to mixed neighborhoods, but largely this is not the case. Desegregation is actually lessened by forcing people together in the name of desegregation. This creates opportunity for positive change, but usually results in cliques forming by race. This does not help desegregation, but rather puts students into potential conflict without resolution.