This does make things a little more fuzzy. I wouldn't be surprised if a local professional photographer was already given an exclusive contract to sell photos. I doubt that vendors can just attend an event and sell something like hot dogs for instance without some kind of agreement.
That would mean the school would have to notify EVERY attendee with a camera that they are not allowed to sell photos due to an exclusivity contract. The only reason the student is in the wrong is if he were notified that such an arrangement exists and therefore he cannot sell them. Even then, I doubt such an arrangement is enforceable.