I don't get it, never have, but then I think red hair is amazing and always did. I went to a Catholic school and a few of the nuns were Irish and had red hair and in my neighborhood of Latinos and blacks, I rarely saw red hair and thought it was BEAUTIFUL. So much so, that I used to regularly have crushes on red-haired boys with freckles.
I still find red hair lovely and totally don't get the weirdness of bullies, other than the fact that those prone to bullying will find a reason. Red hair. Fat. Too skinny. Too short. Too tall. Kinky hair/curly hair. Hammer toes. Zits. Stuttering. Or no reason at all (as we've seen with some kids bullied almost for the pure sake of bullying.)
Bullies just like to find the "odd man" or "odd girl" out (and red hair is rare) in order to find soemone to pick on to feel superior. Hate, hate, hate bullies.
And I hope she dyes her hair bright red and tells them to F themselves.