Oh Darl,
I'm so sorry, It really breaks your heart when your LO's comes home upset because of some mean-spirited little brats!!! Grrr.
FWIW, my Jess only turned 6 in December & she just lost her first took in February! She is definitely NOT a baby for still having her little teeth!
Kids are kids & yes, kids are cruel. But still, I don't think they are ever too young to learn how to treat somebody elses feelings!
I would have a quiet word to the teacher & see what she makes of it. School is supposed to provide a positive environment which encourages your child to learn to their full potential - not an unhappy place full of bad memories (& kids).
Good luck honey, give your beautiful little girl some from us!
xx