This study uses the first age-period-cohort (APC) analysis of segregation to examine changes in public school segregation from 1999-2000 to 2013-2014. APC analyses disentangle distinct sources of change in segregation, and they account for grade effects that could distort temporal trends if grade distributions change over time. Findings indicate that grade effects are substantial, drastically reducing segregation at the transition to middle school and further at the transition to high school. This does not substantially distort recent trends, however, because grade distributions were sufficiently stable. Black-white segregation was stagnant overall, while Hispanic-white segregation declined modestly. In both cases, declines across periods were offset by increases across cohorts. Further analyses reveal variation in these trends across metropolitan and non-metropolitan areas, regions, and areas with different histories of desegregation policy.