This study addresses the effects of secondary schools and classes on language achievement in Flanders, Belgium. The results of a three-level analysis (students within classes within schools) indicate that the group composition at the class level is very important. In classes with a high average initial cognitive ability or a large proportion of girls, the language achievement is higher. These compositional effects are discussed with reference to type "A" and type "B" effects. The analyses show that group composition is more important than educational practices in accounting for differences in language achievement. With whom one is taught has a larger impact than how one is taught. Indications of differential effectiveness of classes related to prior achievement were found, with greater variations in effectiveness between classes for pupils of low prior achievement. (Contains 2 figures, 4 tables and 2 notes.)