It seems more than a little extreme to expel you for being actually sick. Asking for a meeting with you, your parents, and administration to make sure that the school has a clear understanding of your health. (I'd paint a vivid picture of exactly why you can't be in class and exactly how hard you try to avoid gluten so they understand it is not because you want a donut.) If the school is still stuck on expelling you for being sick go up the administration ladder and keep going up.
I have had good results as a last resort with the media. When the newspaper phones them to comfirm that they are expelling a student who behaves well, and does the work, but has a disease that sometimes prevents you from living your life and attending school. They might decide to back down.
Good luck.