This paper employs the difference-in-differences empirical strategy and quantile regressions to analyze the effects of sorting and mixing on the academic performance of high school students in South Korea. In Korea, about half of high schools are subject to the equalization policy (EP), and must therefore passively accept students randomly assigned to them. On the other hand, about half of high schools are in non-EP areas, so students are sorted among schools based on students' ability levels. Two main results emerge from this study. First, sorting raises test scores of students outside the EP areas by roughly 0.3 standard deviations, relative to mixing. Second, more surprisingly, quantile regression results reveal that sorting helps students above the median in the ability distribution, and does no harm to those below the median. (Contains 2 figures and 6 tables.)