Mean Girls

I struggle with knowing what to do when it comes to protecting my kids.  When is it too much, when is it not enough?  When do I let them try and figure it out on their own or step in and take care of the situation?  I struggle with this all of the time.
The Girlchick tends to be more on the sensitive side.  She came home from school the other day and was telling us that her best friend at school has been teasing her.  This isn't the first time, but it really bothers me that at this young age they are teasing each other and being mean to each other.  Where do they learn this from - older siblings, their parents?  I don't get it!  She said that her friend was making fun of her clothes.  That they aren't pretty and nice and don't match.  I do have to say that The Girlchick has her own sense of style.  She doesn't always match, doesn't always have her hair brushed, she loves crazy socks that she will mix and match, loves to wear dresses that twirl, sometimes she will even wear different shoes.  But it is her style.  I stopped telling her what to wear when she was 3.  It was a battle that I didn't want to fight and there really was no point in it.  But the fact that she gets teased for it hurts my heart, I just want to march up to that mean girl and tell her to stop and that she is just jealous.  But how does an adult do that to a 6 yr old without looking really stupid and being a big bully.  I just want to protect my child and her innocence.  I don't want her to have to deal with it and know what it is.  But I know it's not right, I would be doing her a disservice and stunting her and her abilities to cope and deal with difficult situations.  But I struggle with how to do it properly and still protect her.  I want her to hang on to that innocence as long as possible.