(1) What are the effects of school composition with regard to prior math achievement, SES, ethnicity and sex on mathematics achievement at the end of the second grade? (2) Are there differential school composition effects? In other words: are all students affected equally by their school composition or are some specific subgroups more sensitive to their school composition than others? (3) Do certain school processes mediate the association between school composition and mathematics achievement at the end of the second grade?