I've had this happen to me with a girl from my Spanish 3 class, her reputation in school was being a flirt so I didn't take her seriously. However, toward the end of the semester she claimed her feelings were genuine and offered to make out with me before class. I was turned off because I couldn't out why she was so interested in me. She would attempt to footsie me during class, clear interest signs yes but she did it with other boys in school too. Needless to say someone else took her up on her offers and he got his heart broke, so I agree with ZacAdam on this one.
It's hard to call without actually being there and seeing what's happening. It could be flirting or teasing which both lead to interest in one way or another.