A... shocking theory (Spoilers)
I don't think A could be any of the girls. A does some really bad stuff to them so why would they do that to themselves? Even though it's an A team why would another one of the other A's do that to them either. Unless she wanted another A to send her the texts so they wouldn't suspect her? Plus the girls tell each other everything so it would be easy for one of them to be A. This is bringing new theories up to me lol!