GIRL probs?

I like this girl.. shes a freshmen, and im a sophmore. shes pretty confusing, she says she likes me ALOT.. and she teases me about alot of stuff, and shes a big flirt.. she told me at the beginning when we started talking, she was just flirting for the attention, but she says that all changed, and now she really does like me alot, but she does like another upperclmen at school(hes not intereested), and shes still infatuated with her ex, who has moved on with another gf. what should i do?I mean want to ask her out.. but idk what shes going to say, and i really do enjoy talking to her, ALOT..shes says that im one of the most amazing ppl shes ever met, because im really sweet and honest..idk, whats your thoughts? Thanks..