I don't think you're being unreasonable Podmog. If they agreed to accommodate her, then did the opposite then they were essentially playing with her life.
If they hadn't wanted to accommodate her then they should have said so - and you could have decided whether to send her or not.
Schilke - nobody is saying your ds2 can't eat nuts - only that he refrains for the few hours he is in the company of those it could harm. Even on a school day that would leave 16+ hours for him to gorge on them. The alternative is that those w. this serious type of allergy can't ever go anywhere safely. And, yes, of course it is silly for a school to ban nuts if no-one attending has a nut allergy.
Last year I shared a large open plan office w. a woman who was anaphylactically allergic to oranges. Even someone peeling an orange at the other end of the room was enough to cause her a reaction. So I, and the other hundred odd people on the floor refrained from eating oranges at our desks. No-one complained, or felt that our 'right' to an orange outweighed her right to life/good health/a job.