I experienced verbal abuse and sometimes physical from peers in school. I experienced teachers supporting and backing up the emotional abuse at times. I can see now it was my extreme shyness, withdrawal that was the trigger, which likely came because I was stressed from situations at home.
I can tell people that ignoring and turning the other cheek doesn't help a bit. Reaction is not what children seem to be after when they act as a larger group. It is more about social bonding together to exclude another person. It is trendy and hip and feels good to have something to rally around from what it seems to me.
If I had a child who was experiencing this, I would place them into therapy with someone who some expert training in bullying/mobbing. I have no idea how to adequately cope with this.
All the nonsense that parents tell children to make themselves feel better DOES NOT help when you are the child having to face this everyday. It needs a deeper, more professional answer. It's a really ugly sort of problem.

to anyone having to deal with this.
If your child get tagged with the "it's trendy to pick on him/her" then it is not the typical "learning to get on with others" that you've (general you, no one in particular) been programmed to see it as. It's like a witch burning hysteria.