It's a good idea to pack your own lunch, definitely especially with Celiac's, Crohn's, Ulcerative colitis and the likes. I've interviewed for dental hygiene and graduate schools before and usually they provide some variety. I'm unsure if schools can actually accommodate a rather specific diet but surely you can find some of the items edible like salads and such. I have some stomach problems myself and have to avoid certain foods but the few interviews I've had in my day weren't bad. I usually just picked salads with dressing on the side.