You could maybe bootstrap it with a stackoverflow-like point system and by allowing people to ask questions across campuses. There's a lot to like about the Yammer strategy, but the key is (1) bootstrapping it (2) error checking and (3) dealing with the fact that 90% of student just cannot be bothered.

What would the super-over-achievers like in exchange for their effort? Job offers/interviews (companies like conscientious team players)? Bragging rights (a la stackOverflow)?