When I was a student we often played pranks on each other!
One day, my friend and I played the following prank on our roommate: We put a clock 2 hours forward. So, in the morning, when the clock rang all of us got up, but then my friend and I said that decided to miss classes, so we again went to bed. The roommate prepared and went to the University...
When he went out, we put the clock back and fall asleep. In half of an hour the roommate returned, frustrated and angry! But our reaction shocked him again. We said that were sleeping and didn't get up. That didn't see him at all that morning! The roommate began to doubt, then thought that it was his dream... However in some days we confessed to him in our prank and laughed together at that! One day he wanted to repeat it, but we were on the look-out!