Vallhala,
Tell your dd how beautiful she is every day and remind her the bullies are just jealous.
I have a dd with beautiful long red hair and whilst I clearly think she is the most beautiful thing in the world, I do recognise that in fact, the most beautiful thing about her is her hair. It makes her striking and she stands out in a crowd.
I've always avoided the g word as somehow it seems like a more perjorative word but we call her hair red or titian, and both seem lovely words.
Your dd is a little old for this uplifting point(and I've not had time to read the whole thread so may be repeating points for which sorry, as am shattered, in NY, and need to sleep, but wanted to send you good vibes and belief,) but in case no-one else has mentioned it almost every other Disney heroine has red hair, which PROVES it's the best thing ever. I remember counting them with my dd when she was little - check it out. Can't remember them all now, but Ariel is a great example.
I now have my colourist sneakily trying to highlight my hair to match my dd's as she thinks it's so beautiful.
Tell your daughter she is so lucky to have stunning, different hair, how lucky she is not to be yet another blonde or brunette, and how eye catching she looks.
Of course, all of those are reasons why silly bullies pick on her, because they are jealous. So follow the advice of other posters and get onto the school about their rotten behaviour. But in the meantime, tell her again. Yes, she's beautiful. And lucky.