Understanding class size effects on educational achievement remains a preoccupation of
many economists. But empirical results are, to this far, still inconclusive. I use the two-stage
least squares and the instrumental variable quantile regression methods on Lesotho’s grade 6
students maths and reading test scores to estimate, respectively, the mean and distributional
effects of class size. I find strong evidence for putative class size effects on reading achievement,
but not on maths achievement. Using the bounds methods of Conley et al. (2012),
I show that these findings are robust to weak violations of the instrumental variable exclusion
restriction requirement. I also find that a small class size generally benefits the low-ability
students the most.