If they continue with that attitude I would pose a question to them. Is the head of the school willing to make a legally binding oath that everything they feed to your child is gluten free? Knowing that if they are exposing her to gluten, they could be charged with assault? Knowing that you are going to buy a gluten test kit and actually test samples of the food they feed her?
My guess is that they would run a mile from making a promise of that nature. Until they can guarantee her safety, they can shut up about it not being fair that your child gets her own lunchbox. Especially when I'm sure she'd rather have the flexibility to eat what everyone else does without getting sick.
Rant over! Sorry about the attitude, I know you have to go about things the right way, but it's hard enough to deal with this sometimes without ignorant authority figures making it worse! Good on you for sticking up for her and trying to get a workable solution