Your options may be guided by where you will work/live.
ISK is in Okpo close to DSME and the main location for expats.
BIFFS is in Busan and would require a school bus journey of around 1 hour.
Gyeongnam is approx. 40 mins drive by school bus.
Iy is one of the GFRA website sponsors (see left on this page)
The Canadian school has minimum expat children, but is only 15 mins drive from Okpo.
One main concern for most parents, apart from the quality of schooling is the driving (their fecking MAD)so a long journey in my view is not an option for my child.