Quiet Kid: *reaches into bag*
Teacher: EVERYBODY RUN!
Quiet Kid: *reaches into bag*
Teacher: EVERYBODY RUN!
Listen up, buttercup. The joke is that the teacher assumes the quiet kid has a weapon in his bag, because that's what happens in movies. It is funny because you would think the teacher would be more rational. By the way, did you know that your brain uses 20% of your body's energy? Maybe that's why you couldn't figure out the joke.