Despite the voluminous literature on the potentials of single-sex schools, there is no consensus on the effects of single-sex schools because of student selection of school types. We exploit a unique feature of schooling in Seoul, the random assignment of students into single-sex versus coeducational high schools, to assess causal effects of single-sex schools on college entrance exam scores and college attendance. Our validation of the random assignment shows comparable socioeconomic backgrounds and prior academic achievement of students attending single-sex schools and coeducational schools, which increases the credibility of our causal estimates of single-sex school effects. Attending all-boys schools or all-girls schools rather than attending coeducational schools is significantly associated with higher average scores on Korean and English test scores. Single-sex schools have a higher percentage of graduates who attended four-year colleges and a lower percentage of graduates who attended two-year junior colleges than coeducational schools. The positive effects of single-sex schools remain substantial, even after taking into account various school-level variables such as teacher quality, the student-teacher ratio, the proportion of students receiving lunch support, and whether the schools are public or private.