I was actually in this position. You've probably heard it over and over again, but the best way is to be yourself, be natural. If you've already started talking, maybe ask for her skype, facebook etc. If you really like her phone number, just tell her you'd like to keep in touch after school.
My situation is slightly different in the fact that I asked the girl to formal after I started talking to her. The first step is hard and I know that all too well. I'm still a little nervous but I'm easing into it and I'm making progress. Of course, I'm aiming all the way.
But, back to you, there are a number of ways you can get her contact details, some if you're feeling a little... "adventurous".
- Ask her on a date, if all goes well, ask her for her number and say that you'd like to hang more often... something like that
- Get to know her more but talking to her more and naturally ask for her contacts to keep in touch after school
- Ask her straight out
- Ask one of her friends
Some options have a better chance for you to get her contacts but if you're crushing then I'd assume you're a little nervous. Talk to her a little, maybe a "hi" when your passing at the beginning, then ease into it, having a short conversation and eventually you'll ease into talking to her and being confident and comfortable enough to ask for her contacts.