Massive cross-sectional evidence exists indicating that children of more educated parents outperform their schoolmates. However, evidence for causal interpretation of this association is weak. We examine a causal relationship using two approaches for identification within the same data: cousins with twin parents and adopted children. We find no effect of mothers' education on children's school performance using the children-of-twins approach. However, for adopted children, mother's education has a small positive effect. Tracking the work experience of parents during offspring childhood, we find no support that this effect can be explained by a higher labor force participation among more educated mothers.