I back that too (^).
I also think it is because you fear a relationship. If you wanted one, you would've asked her.
It sounds like you like these girls and they like you back. You really want to just be friends, but something is pulling you to do more.
Due to lack of experiance, social problems, or fear, you never push the relationship further.
Eventually the girl and you become flirting friends and it fades away from there.
My suggestion (which is menial because I lack experiance too!): decide how much you like this girl and how much it interests you enough to keep going further. If your curiosity on women stop and you want to play Halo instead, than you need to get out more.
I think that eventually, it won't matter like all other things in High School. HS is just to get out there and have fun. If you don't have alot of fun at parties because of lack of interest, then don't obligate your attendence. In the long run, most men just seem to develop a since of curiosity for women.
If you are undecided, then make friends with her and stop ignoring her. If you start to like her more in all ways, then push the relationship. If it falls back, you'll at least have a good friend
Joseph
Jam it back in, in the dark.