Just tell her you would like to enjoy her company for lunch, dinner, even mini-golf or go-carts. If you can't afford it, ask her to pay her way or take her window shopping at the mall. (Hint: Any public place that offers little or no chance for any sex. Movies are out.) Be the boy-scout ideal for the perfect gentleman. Open the door for her. Offer to escort her to her car. Help her study at the library. Compliment her on her intelligence, her personality. Most importantly, make sure that she understands that you are not interested in having sex with her. The more time she spends with you clothed, the more value she may find in herself (and in you).
You don't need to become her boyfriend, just a good friend that just happens to be a guy.