Further, it'd be interesting to see their eventual success against those who went to the coed schools.
It'd also be interesting to run a self-assessment of the single-sex students vs the coed students asking them a broad array of well-being questions, e.g. their happiness, level of fulfillment and satisfaction in life, etc.
Interesting starting point, but like pointed out flawed to say single-sex schools are better overall purely based on this one metric. The only thing that can be said here is that single-sex schools drive greater admission to colleges than coed schools in Seoul, given the way these schools are administered there.
Things that definitely need to be addressed - what are the differences in the coed vs single-sex schools these people go to? Are the single-sex schools taught differently at all? And if so, how? Why does Seoul have a single-sex vs coed separation in the first place?