Diversity needs to flower in it's own time, not be ramrodded through by government legislation and legal dictate.
While I mostly agree, I think in cases with severe segregation, there's something of a collective action problem that government could alleviate. Imagine a city where the schools are either all-white or all-black (which was basically the case in many cities). Even if quite a few parents would be happy to send their children to an integrated school, they might not like the isolation and conspicuousness of having their child be the
only white kid in a black school or black kid in a white school (or at least be very heavily outnumbered). So no-one makes the first move, and all schools remain completely segregated.
Thomas Schelling's
model of segregation is relevant. The equilibrium can be for total segregation even when racial preferences are mild (and could potentially improve with exposure).