I don’t have a child myself with a 504 plan but work in an educational setting (higher ed) so can speak to 504 plans. Typically all accommodations need to be linked to a specific “need” specified in a medical/psychological report; for instance time and a half on assessments or separate setting for assessments should be linked to needs in the area of focus, distractibility, etc. Easy enough with an ADHD diagnosis. I have never seen in a higher ed setting an accommodation allowing a student to use class notes on assessments if the rest of the class isn’t allowed. I have seen 504 plans that a student can get a printed copy of class notes (either from a classmate or the instructor’s own lecture outline, etc.) especially in cases where writing is an issue. My guess is that boarding/prep schools are similar to higher ed in that they would require justification and evidence as to why certain accommodations are required. I would just make sure your ducks are in a row (testing and diagnosis is updated, needs clearly outlined from a provider) before the fall, just my advice!