Diarrhea

Student A: Did everyone see the diarrhea in the hall?
Students: Ewwwwwww, what?
Student A: I swear, there was diarrhea in the hall. All over.
Mr. Kevin: Student A, please focus on your work.
Student A: But there was diarrhea!
Mr. Kevin: I doubt it and I don't care.
Student B: Who did it?
Student A: I don't know, but it couldn't be a girl.
Student C: Yeah-huh it could be a girl.
Student A: No, it's diarrhea, so it couldn't be a girl.
Mr. Kevin: I don't have the time to explain this to you right now, Student A, but contrary to popular opinion, girls do poop.
Students: laughlaughlaugh

Probably provoked by laughter, an unknown student farts loudly, spawning more laughter.

Student A: Ha! Who farted?
Mr. Kevin: Well, we know it can't be a girl...