used to have the portraits of all the presidents displayed on my classroom walls. was teaching away one day when i noticed that suddenly each one was suddenly wearing a dodgers hat. it was a good prank from one of my colleagues and her son.
i returned the favor on her last day of school by printing out an unflattering yearbook picture of her wearing an angels hat, then taped them all over her classroom.
best one ever done to me was by my roommate and fellow jr. high teacher. he found an old publicity photo of me, xeroxed it, and then the let the kids color it. they turned me into a pirate, doctor, football player, cross dresser, you name it. he then taped them all over my classroom. great surprise to discover that morning, and the kids loved it. still have those pictures somewhere.