She is 15 you are 17. She more than likely doesn't know what she wants, so a 15 yo is going to flirt with who ever she can.....Why? Because she enjoys the attention. If you get involved with her, she prolly will continue to flirt with other guys wether you like it or not.
Do not try to be what you think she thinks she wants. Because like I said earlier she probably hasn't got a clue what she wants. Attention spans don't last very long at that age, so I highly doubt she has any strong attractions towards you. Flirt with her with the attitude of "Can't touch this!" and remember you are a guy and that is what women go for, the heterosexual ones anyway...LOL. So with that, you got what she wants. You make the call!
Forget about wondering wether she still has a crush on you. If she does, why not make her crush you more? If she doesn't it is not going to hurt to try and get her to have a crush on you again. Don't be a fool and rush in when you are unsure about what she thinks of you. Do it when you don't care what she thinks of you.
Impossible is a word only to be found in the dictionary of fools.
Napoleon I