Like the other guy said, you have to have confidence. Don't hide behind e-mails and phone calls cuz then you'll look like too much of a wussy to ask her face to face.
Just say to her matter of factly, like it ain't no thang (cuz it's not to a confident guy), I enjoy your company and find you really attractive, and I'd like to take you out this weekend. Are you free Friday or would Saturday be better?
She already knows you so there's not much point in getting a phone number from her to call her on the phone and "chat." The only reason you need a phone number now is so you can work out the logistics of your future date!
If she says she is busy Friday and Saturday, you can tell she is interested if she makes a counter-offer like, "Well, I'm busy, but wanna get some coffee before class on Thursday?" Or something like that. If she counter-offers, you're in the house.
If she flat out turns you down or says she's busy and doesn't counter-offer then tell her, "That's too bad, we may have had lots of fun."
Women aren't too hard to deal with. Just act confident, don't mince words and never ever act apologetic because you find them attractive!!!
-Nick Viejo.