Once in middle school, I has left behind in a room full of iTouch's so I set an alarm on every single one that said "Steve did it" (Stephen is my name and I HATE being called Steve) they all went off during the middle of someone's speech, (dang, rumors spread like wildfire) and since I hate being called Steve no one thought I did it. BEST MOMENT OF MY LIFE SO FAR.
At a church camp once, in the leaders' cabin, as a prank some of the girls took a bunch of alarm clocks at set them for half hour intervals (the time it usually takes most people to fall asleep again once they've been woken up) and hid them in random spots all over the cabin. So when one rang in the middle of the night, all the leaders would be bumbling around trying to find the alarm clock to turn it off and once they did and went back to sleep, another one would ring. xD It may sound kind of mean, but the leaders were good sports and laughed about it the next day. It makes for a funny story.