A great many adults always told me to ignore bullies and when they get bored they'll go away. Over the last 12 years I've found that when you ignore bullies they do not get bored and go away, instead virtually all of them (girls and boys) are provoked to greater action and eventually physical violence. If these children are below about 13 they're young enough that adult intervention is still perfectly reasonable.
This should have been dealt with decisively by a teacher already and if it hasn't then going to the principal and making sure these teachers are performing the most basic of their duties is not unreasonable. Nobody should ever have to "cope" with constant harassment of this level. What you describe is not minor namecalling or trolling, it's an organized and systematic campaign of torment being carried out.
Chances are there's more going on that you haven't been told of. In the meantime... there is no real way to teach someone to deal with this, soemthing of this magnitude isn't a jarring interruption like short-term hecklers, it's a major and constant facet of her life. Not caring about other people's opinions to this significant of a degree is not something that can be taught to a child, either you're born with the right attitude or you die inside enough that you stop caring.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.