How about you try the "weaning" process during a break from school, like a 3 day weekend, is there some sort of holiday period coming up that you could use to your advantage, start her on a Friday as that would give you four days ?
There is a lot of controversy with adults and addictive substances as to how to best go about gradual vs. cold turkey and get it over with withdrawals, I am not sure of what to say for a kid who won't be able to think "okay, this really sucks that I feel bad now, but in a week it will be worth it." The problem with schools is that they are also full of other kids eating junk food, and sugary gluten crumbs, and milk products, and full of cross contamination from things like play- doughs and snack times. Also, she may have issues with other foods or food colorings or additives, that are not going to be obvious until you get to being gluten free.
I am trying to guess that she is getting cravings and it is no fun at all. Cravings are caused sometimes by malnutrition. You have to be really careful substituting the gluten free items at first for the regular ones, because the sensitive tummy is going to be reacting to these new ingredients. Soy flour and milk casein (the protein) can be a big problem. Say somebody switches out regular cereal for gluten free cereal with one iffy ingredient you don't know about yet, and then uses Rice Dream on it which is contaminated with barley during processing, even tho this company DOES NOT admit this on the label, or they use Soy Dream on it, instead of a nut milk or coconut milk that doesn't have it. Then you make this problem you didn't realize you had, worse for awhile.
Can you get some green vegetables into her somehow, even if you have to make it by putting spinach or parsley into a fruit smoothie with berries and bananas ? There is nothing that fruit and vegetables will not make better. The other thing that is important is can you possibly get some good fats into her in the form of coconut milk, nuts, eggs, avocados, olive oil ? Both of these things are good for hunger pangs. For meals, other than gluten, I'd let her have anything she wanted, as long as it was gluten free during the shakedown phase, as long as the meal had a serving of fruit and/or vegetable, and some form of fat. For example, rice pudding for breakfast, made with leftover cooked brown rice, coconut milk, choice of sweetener, and some fruit. Don't try to do this low fat, it doesn't work, especially for kids who need some dietary fat for brain development. You can do the same with mashed cook sweet potato, or canned pumpkin. If you think the kid just can't do broccoli, try putting some cinnamon on it. You can also do a "teriyaki" sauce with coconut aminos for the soy sauce, pure gluten free vinegar, and orange juice, and bake some chicken wings in it. Then serve it with rice and some pineapple for a "Hawaiian breakfast." You can "bread" chicken strips in almost anything gluten free, such as flours or nut or seed meals, after dipping it in water, egg, or chia seed gel, (chia soaked in cool water sets up) and bake them in a pan with olive oil, and serve them with home fries (regular or sweet potato) for breakfast. You can also do a dairy free buckwheat pancake. Three tablespoons of buckwheat cereal ground up in a coffee grinder, add water to make a thick pancake consistency, along with a spoon of oil, a pinch of salt, and a generous pinch of baking soda and small one of cream of tartar, salt, and optional spice and sweetener. Does not need egg to set up. Fry in olive oil, (might make 2 smaller pancakes) sprinkle with cinnamon and you have something that is very fast to make and even tastes good. You can add a fruit to it like blueberries sprinkled on top, or bananas, and have an egg or bacon or some chicken on the side. It is also good with peanut butter or chocolate chips (Enjoy Life is allergen free chocolate). Leave out the sweetener and it's a flatbread. You can also make excellent brownies out of canned, well drained, and rinsed and mashed black beans (google the recipe "Black Bean Brownies" on the internet, it is a cross over from the regular cooking world). "Cookies" can be made from peanut butter (flourless) and you can add coconut flakes to them.
Any of these ideas will also, of course, work for dinner. I am trying to think of sweeter stuff, but higher fat and protein, because the kid will be wanting sugar.
If you are giving her the big dose of gluten in the evening, then by the next day she's going to be in full reaction to it, and the teachers get to deal with the consequences.
While you just about have proved, along with the positive celiac blood test, that This Is The Problem, it might be better to time it so it happens at a different time, like she eats it at school lunch, (and I will bet she is getting some now at school, anyway, via cross contamination) so at least she is at home and you get to see what happens when it hits.