My heart aches just reading this, substitute. There's scarcely anything worse than knowing one of our children is going through this.
I don't know what you say to a child and am glad to see that you've been given some references. I probably thought that I did back in the day! I truly can't remember how I handled this but it did pass.
I noticed this for my daughter at about the same age. Little girls can be especially mean to each other and it's crushing. I did observe that the role was passed around from week to week and alliances shifted just as often. That's small comfort, I know.
I remember kids from my school, though, who became a scapegoat for as long as they were attending and I often think about what may have happened to them. I can't imagine living with that kind of emotional torture day-in, day-out for all those formative years.
And that comment about the teacher can be true. I've seen mean-spirited teachers who allow and even support that kind of situation. So if you aren't getting staff support for your issue there I imagine it's time to speak up.
Some thoughtful teachers are at a loss for tactics against bullying. Some think to avoid appearing to favor the bullied child that it will go away for fear of adding fuel to the fire.
You are fortunate to be parenting in a time where this problem is increasingly recognized and taken seriously so there will be someone, surely, at the school who will hear you out should this begin to be a persistent problem.
I do know that some of the kids who had a tough time in school turned out to be very gentle and appealing humans as though tempered by fire. Sometimes I wonder how any of us got through school without being permanently scarred.
I'm not prone to give advice but I can manage this much: Not responding to the bullying sometimes will work after a veeery tough trial period.
Comfort and courage, susbstitute, from another parent. The same for your daughter.