This will probably work in a school or work environment.
Get a piece of paper and draw up the tic tac toe game. Fold the paper and give it to the girl. She will either think that you want to play tic tac toe, or she will think you want her number. The reason for the number thing is that the tic tac toe lines kinda look like a # so if she denies the phone number you could just say be like "what?, no i just wanted to start a game of tic tac toe."

you could vary the size of the # or maybe draw a x through one of the squares but that's all opitional. I tried it twice, and both times the girls gave me their number, they didn't even think maybe it was tic tac toe squares. lol

Dude you're over thinking it way too much. Here's what I'd do if I were you.

Me: you seem like a real smart girl. I imagine you're pretty good at mind games and puzzles

Her: sure/depends on the puzzle/ no i suck/ whatever

Me: I got one here, let's see if you can answer it correctly.

Get out your piece of paper and draw this

_ _ _ - _ _ _ - _ _ _ _

Me: if you get it right, you'll get a nice surprise

Rolf Perfect