As more primary and secondary students worldwide seek after-school tutoring in academic subjects, concerns are being raised about whether after-school tutoring can raise average test scores without widening the variability in student performance, and whether students of certain ability levels may benefit more than others from after-school tutoring. To address these questions, I compared the distributions of student performance across countries with differing levels of participation in after-school tutoring, while controlling for country-level unobserved heterogeneity using a fixed-effects model. Participating in either mathematics or science tutoring after school is found to raise national average performance without widening the dispersion in student performance. In science, low-performing students benefit more from tutoring than do high-performing students. In mathematics, high-performing students benefit more from tutoring than do low-performing students.