The proportion of low-achieving children in a class can affect the progress of individual pupils in that class. Having a large proportion of low achievers in a class could slow down growth in cognitive achievement but, might also boost such growth, due to the effects of specialist teaching geared to low achievers' needs. In a longitudinal study of 8,684 pupils aged 9-12, cognitive and social-emotional development were analysed. In classes with a comparatively large number of pupils with migrant parents, the increase in pupils' mathematics achievement was smaller. In classes with a comparatively large number of pupils with parents with a low level of education, the increase in pupils' achievement in Dutch language proficiency was smaller. No relationship was found between class composition and either well-being at school or academic self-concept. The effects of class composition appeared to be differential for different groups of pupils. (Contains 3 figures, 7 notes and 5 tables.)