Today's typical minority student attends school with fewer whites than his counterpart in 1970. This apparent resegregation of U.S. schools has sparked outrage and debate. Some blame a rollback of desegregation policies designed to distribute students more evenly among schools; others blame the changing racial composition of the student population. This study clarifies the link between distributive processes of segregation, population change, and school racial composition by framing school segregation as a mode of social closure. I use a novel decomposition approach to determine the relative contributions of distributive processes and compositional change in the apparent resegregation of schools from 1993 to 2010. For the most part, compositional changes are to blame for the declining presence of whites in minorities' schools. During this period, whites and minorities actually became more evenly distributed across schools, helping increase minority students' exposure to whites. Further decompositions reveal the continued success of district-level desegregation efforts, but the greatest barrier to progress appears to be the uneven distribution of students between school districts in the same area. These findings call for new research and new policies to address contemporary school segregation.