This isn't an accommodation, or ADHD specific, but my mum used to teach exam skills to high school students, e.g.:
During reading time have a look at how the exam is set out, how many sections, the weighting of the sections, how many points per question, etc. Make some rough mental calculations about how much time you should allocate per section/question. Note to anything you think you might have trouble with.
Once you're allowed to pick up a pen, write down the start/finish time for each section, so you know how long you have, eg., Section A 10:00am-10:15am, Section B 10:15am-10:40am, etc., making sure to allocate time at the end of the exam to review/correct your answers or and work on anything you couldn't figure out at the time. Then go back through and read each question again, underlining any important terms to make sure you don't make silly mistakes, e.g. mean v median, how v why, (making up examples, but you know what I mean), and marking any big-point questions with a star.
Now go back and start doing the exam. If you come up against a tricky question, only use your allocated time per question, then mark it with an asterisk, move on and come back to it when you've finished.
Following a strict exam procedure like that can help a student to not get stuck on a question and run out of time. When you've finished all the questions, go back and check your working/answers.
Do not leave the exam room early, even if you think you've got everything right. Keep checking for silly mistakes, recalculate everything, make sure you've used the number of decimal points they asked for, etc. All the little things.
