And honestly, it's not like you're going to be bumping into people with their checkbooks open, or something, at startup school.
Having your friend present for might be ethical, assuming your friend states that it isn't his business plan that he is presenting upfront to the judges. But as a judge, my first thought would be 'If this person's business plan is so wonderful, why can't he even give it the importance to present it himself?', regardless of the excuse. So it seems kind of scattered to try to do both things simultaneously in this fashion.
Okay, here's what I would do: put the business plan competition out of my head, and focus my energy on making the most of the startup school experience. Forget about the prize money - you'll have lots of opportunity to make money, but fewer opportunities to get the kind of experience / contacts startup school might bring.
Good luck.