There have always been multiple justifications for desegregation - among the most cited are 1) separate schools will always have resource inequalities, and 2) social interaction in the early years can spur social integration later on.
What were the effects of desegregation on its intended beneficiaries - black students - and if these effects were positive, what mechanisms explain these effects? Sarah Reber, a UCLA economist, wanted to know, too. In this important paper, she found the following:
In Louisiana, substantial reductions in segregation between 1965 and 1970 were accompanied by large increases in per-pupil funding. This additional funding was used to "level up" school spending in integrated schools to the level previously experienced only in the white schools...the increase in funding associated with desegregation was more important than the increased exposure to whites. A simple cost-benefit calculation suggests that the additional school spending was more than offset by higher earnings due to increased educational attainment...the results of this paper are consistent with earlier work suggesting that desegregation improved educational attainment for blacks and sheds new light on the potential mechanism behind this improvement in Louisiana: increased funding for blacks' schools.
Funding is one mechanism, but teachers are another. At AEFA last weekend, Sunny Ladd presented a new paper on teacher working conditions, in which the outcome of interest was teachers' intent to leave the school. She found that net of working conditions in the building, teachers were more likely to express intent to leave in schools with higher proportions of African-American students. This finding is also buttressed by a huge literature on how teachers' decisions about where to work are shaped by the racial composition of the school. It is simply more difficult to attract teachers to schools with high proportions of African-American students.
If you're interested in the non-cognitive side of this issue, check out this study by Amy Stuart Wells. (See her report "How Desegregation Changed Us.")