Lucifer dont you ever just walk passed one of their desks and slam down on it with something to make a very loud bang to freak the shit out of them and wake them up? or i dunno say draw on their faces with a semi-perm ink pen whilst theyre asleep? I think you could get away with it AND it would ease your boredom 
lol. i didn't, but i used to terrorise kids doing GCSE exams, when i invigilated them. if i saw a kid beginning to become stressed or freaked, i'd go and tell them to do hand-stretching exercises, so they didn't get cramp. it meant flicking their fingers about for a bit. eventually, they'd look at me pleadingly (i always moved away from them), and ask if they could stop. i'd wander back, and ask them if it felt better. they always said "no". then i'd just grin, and say it wasn't to make them feel better, it was to entertain me, cos i was bored, and they looked really stupid farting about making daft shapes with their hands. they always went to have a go at me, and i'd walk off sharpish, but they'd forgotten to be wound up - it alwyas worked.
the other thing was to find the kid with the best sweets to snaffle. that stopped them worrying about the exam, when they saw me bearing down on them, with my eyes on their sweets, and an evil glint in my eye.
