I'm so sorry your daughter is being ridiculed, D.. As a retired teacher, I can assure you that kids WILL find the smallest difference (glasses, braces, pale skin, dark skin, etc.) and make a mountain out of a negative molehill with it. I wonder how aware of it the teachers are?
Nine year old girls can be ESPECIALLY snotty to each other, and they look for ways to cut other girls out of cliques and groups in order to prop up their own self-images and "status" It usually gets worse as they get older, and by high school it can be extremely destructive, EVEN TO THE POPULAR GIRLS.
This is the seed of HATE crimes, which can lead to real tragedies more and more frequently in high schools. This is also the female version of BULLYING and should be exposed as BOTH in the classroom environment. When we connect the dots for them, they often re-think their unkind behavior.
It's a tough balance, also, with young girls (I had two) when appearance starts to take on such importance among their own peers. We certainly don't want our girls looking for their personal worth in things they can't change and which, in 40 years, will not matter one bit.
But getting off of my soapbox for a minute, I also think that in this particular case you could take your lead from your daughter. How hard is SHE taking it? If it's very hurtful, by all means, let her pluck or wax her eye brows. After the first time, she may decide it's not worth it! LOLOL
My own girls have very heavy brows, and even as young adults (21 ans 23) one chooses to shape hers and the other could care less. They are both popular and beautiful to their peers, because those things tend to lose importance after high school and everything about your body is more properly proportioned.
Please check out my profile, D., and drop me a line if you think your daughter would find some solace in having an older "sister" for a new friend. We have some wonderful, kind and hip young adult women who would love to spend time with a "little sister" during the week. And that can make all the difference for a girl just starting those tough pre-teen years. It certainly did for mine.
God's best to you both ~ V. J.