Purpose: There is little research on the role of school composition in young children’s behaviour. School composition effects may be particularly important for children in disadvantaged circumstances, such as those growing up in poverty. We explored the role of school academic and socio-economic composition in internalising problems, externalising problems and prosocial behaviour at age 7 years, and tested if it moderates the effect of family poverty on these outcomes. Methods: We used data from 7225 7-year-olds of the Millennium Cohort Study who attended state primary schools in England and for whom we had information on these outcomes. In multiple membership models, we allowed for clustering of children in schools and moves between schools since the beginning of school, at age 5. Our school academic and socio-economic composition variables were school-level achievement and % of pupils eligible for free school-meals, respectively. Poverty (family income below the poverty line) was measured in all sweeps until age 7. We explored the roles of both timing and duration of poverty. Results: The effects of poverty were strong and robust to adjustment. School socio-economic composition was associated with individual children’s internalising and externalising problems, even in adjusted models. School composition did not interact with poverty to predict any of the outcomes. Conclusions: Neither the academic nor the socio-economic composition of the school moderated the effect of family poverty on children’s behaviour in primary school. However, children attending schools with more disadvantaged socio-economic intakes had more internalising and externalising problems than their counterparts.