i used to pull these hilarious pranks on my 8th grade science teacher. the guy was so old, the funniest thing was that i'd do something to him and he'd never notice. For an example, i would stick gum on the icons of his computer screen while he was away, he didn't even notice it until 2 days later.
One time I was playing with my ipod and i had an app that played phone sounds. I was expecting him to get angry that someone had their phone out, but instead the old man WHILE ACROSS THE ROOM took his phone out, and i swear to god, checked to see if someone called him
There's also that time i flicked pennies at him, the first couple shots hit the board behind him, the next shot hit the tip of the sink on his desk (science classroom) and it made an extremely loud DING noise. he didn't even hear it. Then i flicked another one and it landed on his shoulder, he didn't even notice it.