I've had many gfs. (not a whore)
But i have experience with girls.
As mentioned above, walk up to her and say.
"hey can we talk for a momment,.....in private" you asked her to walk around with you around in school, and start talking normally, ask questions just to start off good.
"how are you, how you been sinced we talked?"
After that just jump to the point. Hey remember when....finish it yourself when your with her.

If she acts like "WTF" just play it cool plane and simple, may i talk to you privately.
Over all of that you will know what to do in that moment.
GOOD LUCK