I messed up, how do I fix it?

I'm 16 and the girl I screwed over is 15. I'm such an idiot. She was all for me, she cared for me, she took care of me and I just brushed her off. She believed in me when nobody else did. I was a slacker all last year. No other girl pushed me like she did. She kept me focused in class when I was slipping, she cheered me on in track when everyone else was ashamed to. I did her wrong. I really did.
In class, when I needed her, I kept her close. I sat by her every day. At first, I'd treat her with respect and treat her right. I helped her carry her stuff, walked with her to class, did little favors for her, gave her hugs, everything. But then my friends started picking at me about it and that's when I slipped up. I ignored her in the halls, dissed her hugs, etc. I knew she'd still hold on to me so I kept doing it. Then, one morning, she walked up to me while I was at a table full of my friends. She said hi to me and I dissed her. All my friends were like "Ohhhh!!". She was obviously upset. She yelled "Well, f@#$ you, too, then!" and walked off.
We got to class, I sat by her and she didn't look at me the whole period. I knew she was through with me when I asked her about the homework and, without turning, she blankly said "You should've got it yourself." I admitted that I should have and she didn't reply.
I thought about talking to her the next day but, the teacher assigned seats for everybody and separated us. Now some other guy sits by her and I wonder if she still thinks of me, the guy sitting in the back of the room. What should I do? I know it's hard to convince a female to come back when her mind is made up