How to start conversation with someone I like?

I'm a junior in high school and I'm in a freshman gym class (long story), anyways, there's a girl in there who I think is really cute and I would like to get to know her. Without talking to her, it's obvious she can tell that I'm athletic, (we have gym stats and I lead in every category other than flexibility). Other than that, she knows nothing about me, and I know nothing about her. Because of the grade difference, we don't have too many friends in common either. I don't want to tell her right off the bat that I like her or that I think she's pretty. Ive tried that before, it never works. All I want is to start conversation and go from there. If things go well, I'd like to take her to prom, which is soon, so I don't have much time. Thanks.