Can a principal keep a student from competing if they have a medical condition?

I am a junior in highschool and have been suffering from chronic migraines. I have missed a lot of school due to them. I have seen a doctor and been given prescription medeicine that doesnt work. I have also been seeing a chiropractor for at least 3 years now. The school has documentation from all of this. This has caused me to become very behind in school. I activley compete in FFA and hold offices at a high level as well as show cattle. Now it has turned into my principal isnt going to allow me to compete in any events, not even showing and selling my cattle wich are a $3000 investment. I am a strong academic student but i am now being forced to go to an alternative school because of this. Is he actually allowed to do this?