Due process takes a long time and will cost $$ for a lawyer. You can do this without going there. Remember, the law is on your side, and many, many schools are providing appropriate accomodations. Many of us DID have to fight administrators for them, but we ARE winning due to medical information, law, liability, deaths in school from life threatening food allergies proving the worst case scenario does happen, etc.
Think of it this way, if you DOCUMENTED your discomfort with the principal's subjecting your child to risk, and a reaction occurs, he is going to be liable. Right now he is trying to avoid documentation to avoid liability. But if you document that he is putting your child at unnecessary risk (by serving allergens in the classroom AND going against the doctor's order), then you proved that he must TRY to provide a safe environment or the environment is surely not safe and he is clearly liable because he didn't TRY to accomodate. Administrator is at MORE liability if he doesn't TRY. If he can show he did everything he could and the child has a reaction, OK...but if he didn't and the child has a reaction, he is going to be liable. By documenting, you shift the liability back to him.
Come to think of it, good idea to have your letter re: risk NOTARIZED.