I thought I posted this, but apparently not.
Useful Dear Colleague letter and resource guide regarding ADHD, released July 2016 (this is the original document, referenced in the link in blackcat's post):
http://www2.ed.gov/about/offices/list/ocr/letters/colleague-201607-504-adhd.pdfA paragraph that might be particularly relevant here (emphasis mine):
"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.44 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.