Was in a psychology class in college. Started to go over sexual deviancy. Professor told us that we could discuss anything because he had seen or discussed it all. I snuck in early the next class and then posted a bunch of stuff from old porn magazines on the board and then pulled the overhead projector down over it. The class before us always left the screen down. Then I went outside and came in after most of the class was already seated. That way they wouldn't know it was me.

Professor came in, put the screen up, and then stared at the board for a minute. Everyone burst out in laughter. Then he left for about 20 minutes. When he came back in he brought the dean of the college in to give us a lecture and state that if he finds out who did it they would be expelled from school.

They never found out. :-P