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 base. The results show few between-school effects of school composition and offerings but important within-school influences of curriculum tracking and coursetaking. In most cases, the difference in achievement between tracks exceeds the difference in achievement between students and dropouts, suggesting that cognitive skill development is affected more by where one is in school than by whether or not one is in school.