Don't

You don't warn her. You're so close, you cannot risk her letting go or anything. It doesn't take you long until you erupt inside her mouth and fill her with cum. She chokes and moves away from your cock in shock.
"Oh my gosh!"
"Sorry," you moan.
"It's... ok. That happens I guess. Uh... wow... I liked that. I really did."
"Thanks," you say and sit up. "We should definitely repeat that. If you like I'm open to more:"
She smiles. "Sounds interesting. I'd like you to come back too. i guess that was a lot for one day, i need to be alone for a bit."
"I totally understand. I'll be heading home. See you tomorrow in school."
She smiles hearing those words. "I'd love to see you tomorrow!" She clearly isn't used to those words.

You walk home. You arrive and it's already very late. You head straight to bed.