Forget the old girl, she sounds crazy if what you're saying is true that she was "hanging out with" and "kissing" other guys. She probably did something with her Ex and is projecting on you like you're a damn screen. Dump her. If not because she's crazy because she said:breadlord said:*Snip*
This comes from the idea that the person who cares the least in a relationship has the most power. And in a double dick move, she turned it into a game. And for the cherry on top of the dick move Sundae, she can't even formulate the proper grammar which means either A) she's dumb as a post B) A victim of the public education system (though with the internet people can find good grammar somewhere so victim becomes an almost obsolete at least quite relative term in this sense) C) she was was wasted as hell (because speech dies first) which would make her even less interesting/admirable. Watch the most recent episode of Community (find it on Hulu). It'll make you laugh and brings up a good point about alcohol.This is a contest to see who can care the less
Dump the girl, don't "go out" with the band girl. Become friends first (so you can avoid batshit this time) and if something happens that goes beyond standard friend category (I think you know to what I refer), then she may be good girlfriend material.
Whatever happens, Godspeed. Try to survive highschool with some sanity left.