Uncovering the effect of school racial composition is difficult, because racial mixing is not accidental but instead an outcome of government and family choices. Using rich panel data on the achievement of Texas students, we disentangle racial composition effects from other aspects of school quality and from differences in abilities and family background. The estimates strongly indicate that a higher percentage of black schoolmates reduces achievement for blacks, while displaying a much smaller and generally insignificant effect on whites. These results suggest that existing levels of segregation in Texas explain a small but meaningful portion of the racial achievement gap.