Lie to her

"Hush Emily, be calm," you say soothingly, bending kiss her gently. "You're not going to get pregnant."

"But I could! And I'd be in so much trouble!" she says anxiously.

"I've had a vasectomy sweetie," you lie, kissing her again. "I can't get you pregnant. I don't have any swimmers to find your eggs."

"Oh that's fantastic Mr. Sharp," she sighs with relief. "My parents would be really mad if I got pregnant. They sent me to an all girls school because there weren't any boys here."

You smile at the thought of her parents delivering this naïve young morsel to you and kiss her gently again, as your mind runs through possibilities.

"You're an adult Emily. You don't need your parent's permission to have sex. You're a big girl now."

"Yes Sir!" she giggles happily, squirming under you. "You made sure of that! You made me a real woman! Thank you so much Mr. Sharp."

"You're welcome Emily," you smile. "Now you better get going. We'll do this again soon, I promise." You pull out of her, seeing thick curds of your potent cream ooze out of her pussy before she straightens her clothes and stands up with a smile. You kiss her again and pat her on the ass as she scampers for the door, looking back at you with a happy grin as she leaves.

You smile to yourself as you finish straightening your clothes, amused at her gullibility, and not caring for a moment whether you've knocked the stupid little bitch up or not.