This article examines whether tracking students for instruction can have a differential effect on student achievement across schools. Two mechanisms are posited for possible school effects: (1) school differences in the determinants of track placement and (2) school variance in the opportunities for learning presented to students by track. I argue that track placement is influenced by characteristics of a school's track structure, by assignment criteria, by the flexibility of track membership, and by a school's scheduling priorities. Differential growth in achievement within track level across schools is explained by school differences in track size and homogeneity, in the quantity and quality of instruction within track, and in the determinants of academic achievement. Longitudinal data from two cohorts of more than 4,000 students provide empirical evidence of school differences in tracking practices and tracking effects on learning. Policy implications of the results for the school choice debate and for organizational and pedagogical practice are discussed.