Why can't the school accommodate someone with dietary restrictions? For example, how much trouble would it be for them to leave the cheese off of one pizza, or substitute with a vegan 'cheese' that you send along with her? This annoys me, LOL.
Okay, here's what I would do. Make a small-sized pizza crust, pre-bake it and then freeze it. Mix your sauce and/or toppings in a container. On pizza day, take the frozen pizza and the container of toppings to the cook--all they have to do is dump the toppings on and bake.