Ok this isn't really a joke, but it was funny as hell at the time.
We had all school mass today, and the principal talked to us for a while after it because we still technically had another 30 minutes of school.
Principal: And now, to remind the girls about their purses, as this has been a problem in the past. Your purses may be no larger than your......
*Teacher that manages the sound accidentally plugs out mic*
Teacher: Whoops, sorry!
Principal: Looks like his purse is bigger than his largest textbook!
Student Body: OoOoOoOoOoOoOoOoOoOoO hahahahahaha
If that doesn't sound funny, it was actually hilarious at the time