I have ADHD and so does my son. Mine is a recent diagnosis however, and I didn’t know about it when he was littler. (He’s 13 now.)
The first thing I’d tell you is to drop the rope about behavior in school. Let the teacher handle it. You can’t fix his behavior there. You can talk until you’re blue in the face, but consequences at home or talking at home is simply too far removed from the moment that he’s having at school. So, just let the school behavior go. Don’t let it impact you.
As for the rest, I do recommend learning a variety of breathing exercises to help you calm down. There’s box breathing, 7-1-8 breathing, belly breathing, etc. Learn them and mix them up to help you calm down so you react to his behavior from a point of wanting to train him into better behaviors rather than reacting to how he’s making you feel.