Fifteen year olds that are "sexually active" are just sluts, why would you (a 20 year old) or anyone else want to have anything to do with her? She is trouble with a capital "T" and obviously knows how to push your buttons. Get away and stay away from her, she is just wallowing in the barnyard and will continue to do so.
What are you doing for yourself" school, job other frirends, and why on earth do you run from her house to all your "friernds" to drag them into the nonsense. You all have better things to do. If you're afraid of being alone at 20, get use to it because you'll find yourself alone through both the best tijmes and the worst times of your life. Get some backbone and do something good for youself whenever you're alone.