This paper uses biographical data from Dakar and Yaoundé, two large African cities, to study the link between number of siblings and school attainment. The data describe all fertility events experienced by parents and the number of siblings of each child over time. The average family size effect is estimated first. The family size effect at different ages is then estimated. The results show that, in Dakar, both the overall and age-specific effect of family size on education are negative and statistically significant. In Yaoundé, the overall effect is not significant, but negative effects at some schooling ages (between 14 and 17) are observed in this study. Finally, the negative impact of family size on school achievement seems to be driven more by elder siblings than by younger ones.