Jenna
Cece
maybe Alison or her twin

I don't think Jenna is in the A team, she isn't really a BFF with girls, but isn't she the one that warned Emily about maya's cousin??
I think Melissa or maybe Cece but I'm not sure...

OO yes how I could forgat Melissa but in book her twin was Jenna and she was the one who killed her. But in series everybody´s got a motiv Jenna, Jaspn probbaly someone from N.A.T. club ..

I just don't like Melisa she is such a bitch, she hates her sister and her friends, maybe there are 2 A-teams. not so evil (if Ali is the red coat I guess this one would be less evil, she still wants to protect her friends right?) and the evil one (I guess Melissa, Jenna and Shauna-in the third season finale they were all together and talking about something) so it's possible that the red coat-Ali was coming to that house to tell her friends that she's still alive (?) but those three hate girls so much that they want them dead and they put house on fire. Ali somehow saves them but she doesn't reveal herself. and the -A message in the end is from the other A-team

(i know it's a pretty lame theory)