Open fire!

Seeing as this is probably a once-in-a-lifetime chance, you're going to go out with a bang. You pull your cock out of Sister Holly's mouth as gently as you can. She looks up at you with confused and disappointed eyes. The perfect target.

The first spurt goes straight across her nose. The second lands on her hair. By the third she's figured out what's happening and recoiled back, presenting her magnificent tits as a target. You fire shot after shot across them, each weaker than the last. Soon the poor girl is covered in your spunk and not really sure what's happening, except that she doesn't like it.