.
Actually, it has to be documented that you have a disability and that you need accommodations. Just because you have a diagnosis, it does not mean it affects your work or your school.
Not all diagnoses are disabilities! ADHD is, but you have to be tested to see how it affects you, personally.

Since you are in high school, your school might be able to do the testing, otherwise it costs per hour and is usually done at an educational testing facility, with a variety of different professionals.
Can cost several hundred dollars per hour.

This is a good article that mentions ADHD and the types of individuals who conduct the tests to determine if you have a disability.

Learning Disabilities: Types of Disorders and Getting Help