As always, indigo has curated a very useful list of resources. I'll highlight (emphasis mine) one of the critical passages from the OCR guidance linked inside those resources:
(Page 12—Resource Guide on Students with ADHD and Section 504
https://www2.ed.gov/about/offices/list/ocr/letters/colleague-201607-504-adhd.pdf)
"Someone with ADHD may achieve a high level of academic success but may nevertheless be substantially limited in a major life activity
due to his or her impairment because of the additional time or effort he or she must spend to read, write, or learn compared to others.
In OCR’s investigative experience, school districts sometimes rely on a student’s average, or better-than average, grade point average (GPA) and make inappropriate decisions. For example, a school district might erroneously assume that a student with an above average GPA does not have a disability, or has no unaddressed needs related to the disability, and therefore fail to conduct a Section 504 evaluation of that student, even if that student is suspected of having or has been diagnosed with ADHD and receives family provided academic supports outside of school.
In passing the Amendments Act, some Members of Congress emphasized that “it is critical to reject the assumption that an individual who has performed well academically cannot be substantially limited in activities such as learning, reading, writing, thinking, or speaking.”
