Help With Getting Her Back!?

I really really like this girl and I need help getting her back. I started talking to her in the beginning of the school year and we hit off great and both liked each other a lot. After a basketball game night we kissed and I was ready to ask her out soon. But then at a school dance the next week she got asked out by this kid and said yes. My friend told me about it in the middle of the dance and I was really confused. I saw her the whole dance and she was crying the whole time. After the dance she came up to me and was crying and hugging me saying "maybe in highschool next year." I was confused that whole night and didn't know what even happened. She got dumped by him 3 weeks later and right away tried talking to me again. I was pissed so I told her to not talk to me and leave me alone. The only problem is she talks to my bestfriend all the time. So it was like I could never escape from her. So since she talked to my friend a lot about me I started to talk to her again and like her. But her friends for some reason don't like me for some **** reason even though I never did anything to them and she listens to all her friends. But when she's not with her friends she feels free to talk to me. She's really confusing because one day she likes me and the next I'm a jerk. I don't get it. We recently had a end of the year pool party and her and all her friends went. We are a grade apart from age but really same age because I am youngest in my grade. So she had a party at her house and a girl in my grade across the street from her had a party at her house for our grade. But when I got there she was there with some of her friends. She went back and forth from her own party to the one I was at. So later at night at the party we were all sitting around the fire and she wasn't around the fire but could see me from the trampoline she was on. So on purposely I let this girl that was standing next to the chair I was sitting in sit on my lap with me. I don't really like her she was just one of my friends but when she was sitting on me she started to cuddle with me and she(girl i like on trampoline) saw and got jealous and went back to her party for the night. Later that night people told me she made out with a kid at her party because I let the girl sit on my lap and get cozzy with me. I just need help getting her back because I'm sick of this back and forth jealousy but I think a big part is her effing friends telling her to not like me. Since it's summer now I think this would be a perfect time. But if I find out she hooks up with another guy I'm done so i don't know what to do. I can text or call her at any time too. Please Help!