What can I do if teacher will not let me use the bathroom?

First I'd like to say I'm diabetic and I drink lots of water every day to maintain my blood sugar so I have to use the bathroom often. About a week ago, I had to use the restroom during class, so I asked my teacher if I could use the restroom and he said no because I go everyday. I then asked if I could go to the nurse (so I could use their bathroom and let them know my teacher is refusing to let me go to the bathroom), and he said no. I had to hold my urine for over an hour causing severe back pain. When I returned home from school, I had to urinate but I had a burning sensation the whole time while going to the bathroom. I ended up going to the doctor and was diagnosed with a UTI. I was informed that it's not uncommon for diabetics to easily get infections from holding a full bladder for too long. The school and staff are fully aware that I am diabetic. Every teacher is informed by the nurses that I am diabetic. I often go to the nurse throughout the school day to check my blood sugar and I have never had a teacher refuse to let me go.