What should my doctor's note say for my service dog?

I have had my SD for over a year but I never got a doctors note (because it isn't required) but I am going to be going back to public school soon and someone recommended to me that I get a doctor's note to make thing easier. My doctor doesn't have other patients with service animals or ESA so she is not aware of how to write it. So to sum this all up, what should the doctor's note say about my service dog and my disability? Thank you!