This study emphasizes the negative association between the number of siblings and school achievement, independent of socio-economic status. This effect is found for siblings of both genders, but is more negative for older siblings than younger ones. Interestingly, this association is less negative in wealthier families. This could be taken into account for public policies to ensure that all their children benefit from equal opportunities, and to design interventions to support larger families, especially from lower-income backgrounds, to ensure that all children have access to the resources and support they need to succeed academically.