The present study investigated the effects of socioeconomic and ethnic classroom composition on developments in students' motivation, sense of classroom belonging, and achievement. A sample of 722 primary school students completed questionnaires from 3rd to 6th grade. Latent growth curve analyses revealed that the reading comprehension scores of students with a low socioeconomic status (SES) were lower for each measurement in more socioeconomically disadvantaged classes, whereas these scores were higher in classes with more ethnic minority students. In practice, these effects may often cancel each other out. Furthermore, in classes with a high share of low-SES or ethnic minority students, students of all backgrounds showed more positive developments in motivation. These findings contradict commonly held fears that disadvantaged students "bring down" other students in the classroom. The results furthermore highlight the importance of studying longitudinal developments.