An inflatable boy goes to his inflatable school and decides he's sick of school. So he pokes the inflatable school, his inflatable self, his inflatable friends, and his inflatable headteacher with a pin. The headteacher goes to him and says: You've let me down, you've let yourself down, and you've let the school down!
I think I was closer to frowning than smiling when I heard this joke. A 9 year old would find this funny, I wouldn't expect an adult to.