Revenge of the House-mistress
How dare you call me 'Thunder-thighs'! I heard what you whispered to your silly friends as I passed you in the corridor and how they giggled at your so-called wit. Well, this will teach you to make fun of your Housemistress! And the other little twerps can expect the same! You're going to learn respect even if it has to be thrashed into you! Now get your hands out of the way!"
Moral: free speech only goes so far. Making fun of authority figures can have painful results.