hey! AFS actually doesn't have a program to korea.
i'm on exchange right now with YFU.
it was pretty expensive and there aren't any scholarships for it.
the experience is great, but if you come, i highly recommend lots and LOTS of korean practice and a strong will. there a lot of things you may not like here...it's really tough.
learn a lot about the culture [especially about older culture] before you decide to come.
my recommendation is, come when you are in college. the korean high school system is really tough and as a high school student, your exchange program might be a little overprotective and you may not get to experience as much.
college=more freedom=more opportunities.