This paper contributes to the class size literature by analysing whether short-run class size effects are constant across grade levels in compulsory school. Results are based on administrative data on all pupils enrolled in Danish public schools. Identification is based on a government-imposed class size cap that creates exogenous variation in class sizes. Significant (albeit modest) negative effects of class size increases are found for children at primary school levels. The effects on math achievement are statistically different across grade levels. Larger classes do not affect girls, non-Western immigrants and socioeconomically disadvantaged pupils more adversely than other pupils.