Methinks we just found out why 'mean girl' has such a problem with the DD's peace signs.... It's the cool socks.... BINGO!!!!!
Apparently the other girls moms are not going out and spending the money socks, shopping at 'Justice', or they simply do not agree that funky socks should be worn.... Your DD has stepped out of the box, ahead of the pack, become a trendsetter, and has, therefore, made herself a target.
Yep, sometimes it can be as simple as that.
Especially with 3rd grade girls.
IMHO, I would simply tell my DD that there are other girls who are not 'mean girls', and that she can learn to choose better friends.
As long as she is 'engaging' with mean-girl, the battle will continue
Peace