Students in Korea who attended single-sex, as opposed to co-educational, secondary schools showed little difference in their achievement scores.
The Office of Education in Korea allocates placements in general high school randomly. In the capital, Seoul, there is a mix of co-educational and single-sex schools. Similarly, teachers are not allowed to choose which school they work at. If they live in a particular school district (there are 10 in Seoul) they will be allocated to one of the schools in that district.
Using this information, a paper in the Economics of Education Review examines the impact of single-sex schools on student achievement. Over seven years, the author found that any positive effects of single-sex schooling were small. The effect was relatively greater for students in the middle of the distribution of test scores. For students at the very top and very bottom, the impact was trivial. There were also no differences in the students’ choices for further study or in their test-taking behaviour.
Source: Mean and distributional impact of single-sex high schools on students’ cognitive achievement, major choice, and test-taking behavior: Evidence from a random assignment policy in Seoul, Korea, Economics of Education Review (2016).