This study provides empirical evidence of the relation between class size and classroom behavioral climate, in particular discipline problems, in primary school. Discipline problems were conceptualized as consisting four components: 1) students behavior is not geared towards learning, 2) students compromise the right of others to learn, 3) learning situation is psychologically or physically unsafe, and 4) students do not take appropriate care of the physical classroom environment. We utilize variation in class size created by maximum class size rule in constructing instrumental variable estimates. The results show that class size has a negative effect on classroom behavioral climate: discipline problems increase as class size increases. The effect was more consistent, and the estimates were larger when we studied teacher evaluations in comparison to student evaluations.