This paper investigates how pupils' growth trajectories in three language domains (reading fluency, spelling, and reading comprehension) are related to their own socioeconomic and ethnic background and to the socioeconomic and ethnic composition of their primary school. Using multilevel piecewise growth curve analysis, the growth trajectories of approximately 5000 pupils from 170 primary schools were estimated. The results indicate that a pupil's socioeconomic and ethnic backgrounds have the largest impact on achievement and growth in reading comprehension and the smallest impact on reading fluency. Furthermore, school ethnic composition--but not socioeconomic composition--was related to pupils' achievement in all three language domains at the first measurement occasion, though none of the types of school composition was related to growth. The research findings therefore imply that to combat the deficits in language proficiency often found in ethnic minority pupils, priority should be given to investments in individual intervention programmes over policy plans to desegregate schools. Moreover, our study identifies the specific points in time during primary school at which intervention programmes are needed the most.