Little Johnny Jones Joke

Little Johnny Jones is in England more or less what Jaimito in Spain. There are lots of jokes about him. Here's one:

At school

One day the mistress says: "Listen, children, from now on when you need to go to the bathroom, you're going to use numbers: number one, if you need to have a pee, and number two, if you need to do a poo".
Everything goes well for a while:
Charlie: "Mistress, number one."
Bob: "Mistress, number two."
Lizzie: "Mistress, number one."
etc.
But one day Little Johnny Jones puts his hand up and shouts: "Mistress, quick give me a number, I have to fart."

GLOSSARY

the mistress - la seño
to have a pee - hacer pis
to do a poo - hacer caca
to fart - tirarse un pedo