In this paper, we study how parents react to a widely-used school policy that puts some children at a learning disadvantage. Specifically, we first document that, in line with findings in other countries, younger children in Spain perform significantly worse at school than their older peers and – key to causal interpretation – that for children born in winter this effect is not due to birth seasonality. Furthermore, the age of school entry effect is significantly greater among children from disadvantaged families. To understand why, we analyze detailed data on parental investment and find that college-educated parents increase their time investment and choose schools with better inputs when their children are the youngest at school entry, while non-college-educated parents do not.