This study examines the relationships among school composition, several aspects of school and classroom context, and students’ literacy skills in science. School composition is also associated with the extent to which school systems are segregated “horizontally,” based on the distribution among schools of students from differing SES backgrounds, or “vertically,” due mainly to mechanisms that select students into different types of schools. The findings have implications for educational policy that concern the differential allocation of human and material resources and the stratification of students into different types of schools and school programs.