It takes some work, but you seal the deal.

You regain your energy and wait for your opportunity...

As the Dean nears orgasm, you sneak over and go to Kristina.
You whisper your plan in her ear.
She nods.

With a flash, you switch places with her and start to lick the Dean's pussy.

It isn't long before she cums, unaware of the switch (since she had her eyes closed in ecstasy).

When she gets release, she leans down to rub your head and is confused.
She opens her eyes and sees Kristina near you.

"Hey, that was sneaky. Good work with that tongue though- both of you."
"I've picked up a few things."
"I can tell. Now let me pay this lovely girl back."

You move away and she takes Kristina's hand, pulling her onto the bed.

The Dean is now licking her pussy, but also accidentally/on purpose positioning herself in an enticing manner for you...

Once she's comfortable- and with prodding from Ms. Dolore-, you move in behind her and rub her pussy with your cock.

"Didn't get enough of her pussy, huh?
Well...okay. Just don't interrupt me too much."

You smile and insert your cock into the Dean's pussy! She's tighter than you expected.

With pleasure, you start to fuck her as she keeps licking Kristina's pussy.
The latter cums, while the former gets close quite quickly.

The young girl asks for a break and that's your cue to help the Dean turn over on the bed. With room made, you move in top of her and re-insert yourself. She seems to enjoy this quite a bit.

Her air of standoffish, vague interest fades as you fuck again...

She finally cums hard on your dick.
She squeezes you tightly on the inside, as well as on the outside.

You lean in for a kiss, testing the waters.