Personally I think she's just looking for a piece. If she liked you, she'd want to be seen with you around the school, not just finding some place where she knows no one will see. Does she talk to you when her friends are around? If no, then yeah, she's just using you. If yes, then there could be something more there, but she's too shy to really talk to you about it. You need to sit her down and get on the same page. You need to know if you should invest into this fling emotionally, or if you should keep emotions out of it, and just keep it strictly physical.