"Little Johnny Crack-up." joke

A young female teacher was giving an assignment to her 6th grade class. It was a large assignment so she started writing high up on the chalkboard. Suddenly there was a giggle from one of the boys in the class.
She quickly turned and asked, "What's so funny Bobby?"
"Well teacher, I just saw one of your garters."
"Get out of my classroom," she yells, "I don't want to see you for three days!"
The teacher turns back to the chalkboard.
Realizing she had forgotten to title the assignment; she reaches to the very top of the chalkboard. Suddenly there is an even louder giggle from another male student. She quickly turns and asks, "What's so funny Billy?"
"Well teacher, I just saw BOTH of your garters."
Again she yells, "Get out of my classroom!" This time the punishment is more severe, and tells him - "I don't want to see you for three weeks!"
Embarrassed and frustrated, she drops the eraser when she turns around again. So she bends over to pick it up. This time there is an burst of laughter from another male student. She quickly turns to see Little Johnny leaving the classroom.
"And where do you think you are going?" she asks.
"Well teach, from what I just saw, my school days are over!"