Dear Rammel,
so simple, so effective: your suggestion works!
I would the student work at a rather low level from the point of view of the environment (they should do programming, computing, etc.) teachers only are allowed to play with Knoppix features... Thus, your suggestion could be enough, even if I think that the default settings are a little misleading (how many times the students will be wrong?).
I'll think about doing better, but I don't promise I'll succeed...

Thanks a lot,
ciao,
Ugo