Pin her to the desk

You move with remarkable quickness for a man your size, even with your bad knee, and before Miss Fulton knows what's happening you are behind her pinning her to the desk with your hips. "You're playing with fire Miss Fulton." You growl in her ear, "Do you even know what you're talking about?" Monica's full ass is wiggling against your pelvis and your already stiff cock gets even harder as you feel it settle between her buttocks.

"Ohh my!" Monica places her palms on the desk for balance and drives her butt back even harder. "Is this what you wanted to do to the naughty Miss DuPlessis? Did you want to hear her beg for mercy?" Her voice rises slightly and she does a credible imitation of Gabrielle's pretty accent.

"Oh M'sieur, Je suis désolé." Your hand finds her breast through the sundress, much fuller and softer than the ones you had tasted not long ago, but no less tempting. "S'il vous plait M'sieur do not spank my ass!" She spreads her legs a little more and continues her imitation of the French girl's accent. "You can do anything, M'sieur. Anything at all, please don't paddle mon cul!"

"Are you sure Miss DuPlessis?" You ask, perfectly willing to go along with this young teacher's game.

"Mais oui M'sieur, I cannot stop you, Sir." Miss Fulton moans deeply. "I am votre captif to use as you wish."