This paper investigates the effect of family size on educational achievement of 15-year-old students in the Czech Republic in 2003. The authors find that a continuous measure of the number of co-residing siblings has a negative effect on school achievement and that this effect persists (somewhat reduced in size) even when controlling for family structure, parental education, type of school, and year at school. They find no interactions between family size and other variables in the analysis. The main finding is consistent across various methods of dealing with missing data. The authors conclude that given contemporary population trends, stratification scholars shall in the future pay more attention to model specification and shall include family size among the predictors in status attainment models if they want to obtain unbiased parameter estimates.