It's hard to talk to her in those circumstances. Do you ever see her in the halls? I guess you'll have to start by saying hi to her when you see her in gym class. At least she'll know you have noticed her. I know you can't flirt in gym class, but you can still make an effort to say something to her when you do see her. Try to catch her in the halls and get her attention. If you have her name, see if any of your buddies know her or know what classes she's in. You've got your work cut out for you, but you'll just have to do a little detective work.

On the way in/out of the Gym class, compliment her, tell her she looks Great! Ask if you can buy her a coffee, smile and let your eyes flirt!

I am a teen girl and a lot of people consider me pretty cute, but if an older boy walked up to me and said something like "You look really nice today" or "Your eyes sparkle like the sun" or "Dang have you always looked so good?" maybe even "That shirt looks good on you.".... for most girls just being talked to like a person by a boy makes our hearts pound... Just knowing a boy thinks we look cute is great but when you talk to her look into her eyes and smile... Do not look at her body unless she slutty like that... and do not talk to her with her friends around or your friends around... Go by her locker when able and ask if she needs any help with her books... She will prally be so shocked and happy the first time she will prally say no, but then say "If you ever need them carried just ask." Good luck!!
Oh and try to get on her good side.. If you play in the same gym give her the ball when playing and if she makes the basket or whatever tell her she did good...

text her

well i am a teenage girl and i think you should say hi and like the others said..say it and let your eyes do the talking...eventually you can ask her for her phone number..so you two can communicate more than just every other day in gym class..but yeah just let it be known that you are interested in her..but not too obvious..walk her to her next class if you can without being late for yours.