Class size has a hump-shaped effect on student achievement in Greece. Smaller class sizes can improve achievement up to a certain point, but increasing class sizes can have very negative effects. Optimal class sizes are around 27 students, with achievement-maximizing sizes around 15. Costs of hiring and firing teachers play a significant role in determining class sizes. Lowering firing costs reduces achievement, while reducing hiring costs improves achievement and decreases class size. Class size caps can be expensive, especially for smaller schools, even when set above average levels.