We studied the effect of the number and characteristics of siblings (sex, age) on school achievement in several grades (kindergarten, first grade, and fifth grade), in two large French cohorts, with more than 16,000 children. Running linear mixed-effects models, we find that, independently of socioeconomic status, having more siblings is negatively associated with school achievement. We found a stronger negative association between the number of siblings and the achievement in older compared to younger siblings. This finding is in line with the resource dilution model, where families with more children have fewer resources available per child, but contradicts the confluence model (i.e., a child's intellectual ability is influenced by the average intellectual ability in the family). The negative association between the number of siblings and achievement was moderated by family income, with weaker effects in wealthier families.