1. Put your phone in your pocket.
2. Walk up to the girl in question.
3. Look the girl right in the eyes, smile and say, "hey, would you like to get a Coke at Sonic after school?"
If she says, no, I can't, you say, "would another time work better for you?"
If she says no again, smile at her and say, "let me know when you're free. I'd love to spend some time getting to know you."