She rubs it off the board just in time before the children come into the classroom. She's relieved.
The next morning she goes in and again finds the drawing on the blackboard with "penis" written underneath. She rubs it off again just in time.
On Wednesday, same again with the drawing taking up a quarter of the blackboard & she rubs it off just in time.
On Thursday she's horrified to find half the blackboard filled with a cock & balls & she has to furiously rub it off so the kids don't see it.
Finally Friday comes & she enters her classroom with a feeling of dread. Sure enough the whole blackboard is filled with a massive knob & nuts, but this time written underneath is the message, "The harder you rub the bigger it gets."