I wish I could give practical advice, but I have had the same problem… from the undiagnosed parent that I know doubt inherited the ADHD genes from…
Keep in mind, I am an educational psychologist.
So I can do a better job convincing other parents, but not my own.
However, I was diagnosed as an adult, so it is easy for me not to worry what they think.
Maybe you can ask your school’s psychologist or counselor for any resources that they have found useful.