We've had almost all our classes together.
Most Helpful Guy
"Hey [Name] do you wanna go see Age of Ultron on Friday?"
Or whatever you wanna do. It's pretty simple. Get to the point in the first sentence, and make it a yes or no question. Also I'd recommend saying it during a good conversation (or if you're talking about movies there's a perfect time to ask). Or just approach her when she's by herself at her locker or in the hall.
Also just look out for opportunities to ask her to do something with you, if some event or place she wants to go to comes up in conversation (like I said with the movie). Then its more natural and easy.1