my guess is maybe it something what we have here called running start
some people can choose to take classes over at the nearby community college
you'd be in both high school and college at the same time (unless some people decided to do full time running start. They take all their classes at college and as long as they fulfill the high school graduation requirements they get their diploma)
I have a couple of friends who finished with their AA by the end of high school. one friend finished both his AA and AED
edit: is late
typed too much