Can my school do this?

My school denied me 6 of my 8 credits last year because of medical absences and i can not get a medical note every single time im out because i have RA and it weakens my immune system. so i get sick sometimes on the weekly basis and when i get really sick i cant go but my rhuematologist faxed a letter to the school about it and they still denied me, can they do this?is this legal? if anything by not going when im infectious im doing them a favor for not exposing other people to illness