currently going through this with my 15yo DD who we strongly suspect is ADHD/ASD although not diagnosed (school agree, and both myself and her older brother have AuDHD diagnosis)
The school have agreed to treat her as needing ALL the help, she has a laptop, access to a scribe (she dictates they do the writing for her) movement breaks, a separate rooms for exams, however ALL that help cannot make her do the homework, or write anything down during the exams.. her attention span wanders.
She was forecast 8 due to her working grades/engagement in class, but its proving in the actual exams we're fighting for 4s and 5s if we're lucky.
We have agreed a strategy during her last round of mocks that she has extra time to read the papers,, work out which questions are worth the most points, do some prep work (write down on another piece of paper what each questions is asking) and to tackle those first.
We also have some reference cards in the car that before each exam we go over what this exam asks for, whether its just finding evidence, or arguing for/against...etc
Her speaking exams in spanish and english will be done while walking in an enclosed environment as more conversational rather than exam-like to try and keep her attention focussed.
You have to be in communication with the school, think outside the box, try to see what works.
She won't do homework, so she attends regular intervention sessions at school. WE use the drive to school for some revision, its 10 mins twice a day, so we have some revision cards and focus on one or two per day.
At the moment she has a good passing grade in Art & Music, we're managing to hit target in Maths, history is borderline, and English and Science are more questionable
Ultimately, remember, exams can be retaken (her older brother is redoing both his English/maths during college) and results do not define who they are.