Her pussy

You decide that you want to fuck Kelly's pussy. You rub the wet tip of your cock across her slit lubricating it, then you slip your hard cock between Kelly's pussy lips and begin to thrust in and out of her pussy while fondling her breasts with both hands.

Kelly braces herself against the desk as you hammer your dick into her pussy.
You can feel her vagina muscles constricting against your cock. You continue to slowly and rhythmically pump in and out of her pussy until you feel that you are about to come.

Suddenly, there's a knock at the office door. You pause your thrusting.

"Don't stop!", Kelly says, "The door's locked. Whoever it is will go away."

You resume your thrusting. But the knocking at the door continues.

"Yes! Yes! Yes!", Kelly screams. You hope whoever's on the other side of the door can't hear her.

Then you hear the door open behind you, immediately followed by a female voice gasping in surprise.

Did you fail to lock the door properly or is it someone who has a key to the office?

Who is it? The head cheerleader? Ms. Saxy? Someone else? Does she join you and the Coach in a threesome?