apologies for the impulse reply, just read the first line of your post. xd.
She still hasn't crossed the bridge on her "ex". If you still want to be with her, you're gonna end up getting cucked. how deep is your relationship with her? doesn't really matter though, seems like you're not into her, and she's still on good terms with her ex. Just talk to the other girl, you're still in school there's nothing worse that's gonna happen other than some highschool drama