Although it is clear that racial segregation is linked to academic achievement gaps, the mechanisms underlying this link have been debated since Coleman published his eponymous 1966 report. In this paper, I examine 16 distinct measures of segregation to determine which is most strongly associated with academic achievement gaps. I find very clear evidence that one aspect of segregation in particular--the disparity in average school poverty rates between white and black students' schools--is consistently the single most powerful correlate of achievement gaps, a pattern that holds in both bivariate and multivariate analyses. This implies that high-poverty schools are, on average, much less effective than lower-poverty schools, and suggests that strategies that reduce the differential exposure of black, Hispanic, and white students to poor classmates may lead to meaningful reductions in academic achievement gaps.