"Could you do me a favor then?" she says.
Sure, anything, you say.
"Well, my teacher said I could get extra credit if I could get you to take your cock out in public and masturbate for me... would you do that?"
Right here? There are other women around!
"Please!" she pleads with you. "You masturbate for me all the time at home!"
It's true, so you agree, and step out of your swimsuit.