A recent study of school and classroom gender mixes in Seoul, South Korea, found variation in achievement for male pupils depending on whether they attended single-sex schools, single-sex classes in mixed-gender (co-educational) schools, or mixed-gender classes. Boys in single-sex classes in mixed schools performed slightly less well (effect size 0.10) than boys in mixed-gender classrooms. Boys in single-sex schools out-performed those in mixed schools by a wider but still small margin (effect size 0.15). However, girls performed the same in classes of all different compositions.
Source: All or Nothing? The Impact of School and Classroom Gender Composition on Effort and Academic Achievement (2015), NBER Working Paper No. 20722