You can do that easily
there is just not enough time and its easy to get caught
There is no bios password
I changed the entire time and crap on my schools p.c for kicks and giggles.
It was easy to do everything and anything else.
Only thing is, the students all gather and make a big deal of it and thus it gets noted. And you get a teacher detention.
I don't remember anyone caring that we had kbuntu running. Even the tech staff were more of the opinon "cool, but don't do it again" when they saw it. Teacher sure as hell didn't care what OS or program we got work done on.