Replace Rebecca with yourself and hope she likes surprises.

You cock is aching so bad you just want to explode. You tap Rebecca on the shoulders and she stands to get out of your way. Holding your finger to your lips you motion her to move towards the top of Katherine's torso.

"Now keep you eyes closed," you instruct Kat.

"Ok," she mutters while licking her lower lip.

God she is so worked up. It is driving you absolutely mad with desire. You move forward and your cock just touches the bottom of her thighs. Kat jumps a little at the contact and whimpers through her bright painted lips. Looking down you notice your leaving little snail trails across the backs of her thighs. Damn, you must be excited.

Taking your thumb and finger you grasp her clitoris with your firm fingers. Katherine grinds a little while you stimulate her and use your free hand to slide two fingers into her sopping pussy. Her back arches as you continue to tweak her clit.

"What...?" Katherine gasps for breath as Rebecca finally makes her presence known.

Her lips lock on Rebecca as Kat arches again and again cumming from all the attention paid by you and Rebecca on her clitty.

Opening her eyes... Katherine doesn't seem that surprised to see Rebecca there.

"I'm sorry. I should of told you," Rebecca is trying not to laugh. "We kind of planned this out. We've been together for most of the year and we both thought you were the perfect guy to take to the prom but we both couldn't go with you so I agreed to let Katherine go with you. Then I would meet you both here after she led you out," Rebecca is still smiling at you.

"Hope you don't mind?" Katherine winks at you.

"Who me?" you smile. "Not at all."