This paper suggests that students' opportunities to learn may be stratified both between and within schools. Schools serving a more affluent and able clientele may offer more rigorous and enriched programs of study, and students in college preparatory curricular programs may have greater access to advanced courses within schools. This notion is tested with a longitudinal, nationally representative sample of public school students from the High School and Beyond data set. The results show few effects of school composition and offerings between schools, but important within-school influences of curricular tracking and course taking. In most cases, the difference in achievement between tracks exceeds the gap between students in school and dropouts, suggesting that students' choice of courses in school may be even more important for the development of cognitive skills than whether one is in school or not. References and 10 data tables are included. (Author/TE)