↵Note: The estimates are based on representative samples of individuals born in 1967, 1972, 1977 or 1982 in one-school districts. All ability measures are standardized. Average class size in Grades 4–6 is instrumented by indicators for being above first, second, or third threshold of the class size rule. All models include the following controls for school district enrollment in grade 4: fixed effects for enrollment segment; linear controls for enrollment, which are interacted with threshold. In addition all models include the following baseline controls: municipality-by-cohort fixed effects, gender, dummy variables for month of birth, dummy variables for mother’s and father’s educational attainment, parental income, mother’s age at child’s birth, indicators for being a first or second generation Nordic immigrant, indicators for being a first or second generation non-Nordic immigrant, an indicator for having separated parents, and the number of siblings. Standard errors adjusted for clustering by enrollment count are in parentheses. ***/**/* = the estimates are significantly different from zero at the 1/5/10 percent level, respectively.