While there is considerable evidence that children's early ability to understand others' mental states, called "theory of mind," is shaped by family experiences, it remains unclear whether children's social interactions at school influence theory of mind (ToM) beyond early childhood. We tested whether the mean level ("quantity") and/or the diversity ("variety") of peers' ToM influenced children's own ToM. We also examined whether peer effects on ToM were independent of possible confounding variables (e.g., verbal ability, social isolation) and comparable across children with different initial levels of ToM and social status. Four hundred fifty-four 8-12-year-old children completed assessments of ToM and peer and friendship nominations at baseline and (for ToM only) 1 year later. The variety (but not the quantity) of peers' ToM predicted the development of children's ToM over and above control variables. The magnitude of the peer effect was comparable across different levels of children's ToM and between children indexed as socially isolated and those who were not. These findings fit with sociocultural models and highlight the importance of the school environment in the development of ToM.