So if I understand correctly, right now there is nothing in the 504 about late work and you want it added? It seems like them accepting late work should be a pretty standard accommodation for ADHD, or there should at least be something in the 504 that PREVENTS the late work from happening in the first place (like a teacher checks in with her at the start of each class, or end of each class, etc).
I think there is some controversy about accommodations for ADHD such as allowing late work--it can teach the person to be a procrastinator, for instance. So I honestly don't know if I'd want that in a 504 for DD. I would want to focus on modifications to improve organizational ability and supports at the school (such as teachers checking in with child and parent in a way that they don't with "typical" kids).
I do think that they should accept emailed copies of work (for instance if she forgets to bring it to school, it could be emailed as soon as she gets home). That way she's not being penalized for late work when she actually DID do the work on time.
I do think this "wait to fail" approach before adding modifications (when you saw issues last year) is kind of ridiculous. Basically it's their way of saying that they want the new teachers/school to have a say in what they need to do.
