What are some REALLY good pranks?

We are studying Guerrilla Warfare in school, and our teacher said he is going to be pranking us. So, we decided to get him before he gets us. We are going to be pranking him almost every day, leading up to a big, really good, prank at the end of the year. So does anyone have any good pranks? Big and Small are all needed! Please nothing that can get him seriously injured or stain his clothes. Thanks!