I don't think that Jake would have turned out preppy without the war. If nothing else, Marco would have prevented that. Marco is his best friend and doesn't fit into the preppy mold. Not only that, but NOT being on the bball team is discouraging and he is less likely to try out again. Not being within a sports circle makes it less likely that he'll be preppy (not that all preps are sporty, but still).
Then again, to completely contradict myself, I've known people with all 5 of the Ani's personalities within all types of social groups. Preps, nerds, jocks, punks, burnouts, etc. etc. So maybe Marco could have become preppy and still been Marco.