Aside from the constant boredom I get from monotone lecture and repetitive babbling during classes, there are sometimes exceptions, specially when your crotch area is persuaded into desiring thoughts and you start drawing naked women on your notebooks.
Any guy knows (or is supposed to) that sometimes these raids of lust just drive by and there's absolutely no necessity of feeling ashamed because of that. If such happens during class, politely ask the teacher to go to the bathroom...