I received this very useful email from Ken Hirsch:
I looked into the basis for the statement I read on Marginal Revolution that "American schools are more segregated by race and class today than they were on the day Martin Luther King, Jr. was killed". The source that was given did not actually have statistics going back to the 1960s, but the author of the report, Gary Orfield, pointed me to an earlier report, "Brown at 50: King's Dream or Plessy's Nightmare" (http://tinyurl.com/BrownAt50), which did contain a time series for one measurement, "Percent of Black Students at Majority White Schools". There's a graph of this statistic for Southern black children on the cover of the report which I am attaching to this email.
This statistic is quite problematic. Most starkly, in "majority minority" states, such as Texas and California, this statistic measures the *opposite* of integration. The more evenly distributed that ethnic groups are in schools, the lower the the number. If all schools in California had exactly the same ethnic make-up, there would be no majority white schools, so 0% of black students would be in them! Indeed, in Table 11 from this report (p. 27), California is given as the most segregated state by this measure.
The other two measures that Dr. Orfield uses have similar problems. Most of the change in all three are probably caused by the increase in the percent of Hispanics and the decrease in the percent of non-Hispanic whites, not by segregation. By most mathematically sensible measures, segregation has decreased and integration has increased over the last 20 years. See "Measuring School Segregation" by David M. Frankel and Oscar Volij for details: http://www.econ.iastate.edu/research/working-papers/p11808
The original post was here.