What about separate user accounts on the computers?
… your trouble-maker could have his own log-in for his own personalized account, that way he’s only creating a mess for himself, not everyone else.
I used to manage a student/faculty multimedia lab at American University, and that’s how we solved this type of issue – personalized user accounts, instead of jut one student account.
Also, set up a Desktop Background that reminds the students to check their scratch disk settings. We used an “Automator” script that also sent a pop-up reminder to the students when they launched FCP.
You could also grade the students on their ability to follow instructions… in theory, that should be incentive enough for him (others) to do things the right way!