When she around others she goes from my Bff from ignoring me?

I had some time off school and at weekends went out with my friend but i went back for a couple of days and in the lessons she was just talking to this other girl,and before hand i told her i was going back cuz i was really nervous about going back and i asked if I could hang around her at break and she was like persuading me not to go back and saying ow iv got to go liberry and im probably gonna be busy at break.When she is with me alone were close and have a good laugh but when were at school she just goes off with other people I try to invole myself and talk to her but its hard to know what to say when her backs to me talking to someone else so i dont really end up talking to her much and this makes her think im boring and probably wants to hang around with other people more. Im not a good socal person im ok when were alone but its harder when im trying to mix with her friends who i dont even know or speak to.this happend last year she just went off with more popular girls than me but then i had another friend but recently i fell out with her so shes the only friend iv got does she actually like me bcuz surley you go around with you friends in school properly as well as out i get the impression she dont want me to go back to school bcuz ill be a nucance and sum1 she feels she has to hang around with also that she dos'nt really care about me she just feels she should talk to me at school, and when I hang around with her and her friends i dont feel a part I just follow them so im not alone and they just accept me for my friends sake (even though she wouldt give a dam if I never hanged around with her).sorry for going on for ages,What shall I do to keep the friendship going and keep with her if I go back school ????x