Another suggestion - ask the teacher what he/she feels might be the problem. Perhaps some insight can be gained from this. Sometimes teachers can "pair up' children during class time in the hopes that a friendship might bloom. Hope this helps ...
Perhaps she is shy.. sometimes it takes the child to make an attempt at friendship, so speak to her and tell her to ask them something maybe about games or toys if she breaks the ice she will make friends if she is waiting for them well it may not happen ot take a while .perhaps she could suggest a game to play in the lunch break. Like all of us you have to make the first move , if she starts the ball rolling all will be well, the only way you can help her is to ask one of them back for a playdate after school ...