Here's a link to the Register's editorial on the Supremes' decision on the two school racial-balancing decisons, regarding the school districts in Louisville and Seattle. The court said that they used race too much. Chief Justice Roberts' opinion was bold in some of its rhetoric -- "the way to stop discrimination on the basis of race is to stop discrimianting on the basis of race" or something like that. But Justice Anthony Kennedy provided the crucial fifth vote, and while he agreed on the outcome of these two cases, he specifically rejected that sentiment, saying there might be situations where it would be constitutional to take race into account (and even I can imagine a prison riot situation where if the wardens didn't take race or ethnicity into account they could get a lot of people killed).
It's harder to imagine a justifiable situation in school assignments. I would have preferred if the court had just reversed Grutter or said Grutter might apply to college or grad school but not to K-12, but the votes probably weren't there.