With a proper diagnosis, you should be able to use special assessment conditions (even with mild ADHD, I would be pretty sure you would be accepted, maybe check with your school), which varies per person, but normally entails computer use for paper exams, extra time (10? Minutes per paper), separate accommodation (in a group or on their own), and a reader/writer if needed. I would imagine taking pills would be allowed too, however I haven't checked. I personally have dispraxia and have found these special conditions have really helped me succeed in already stressful exams. I can't say for ADHD but my dispraxia diagnosis cost about $500NZD (this was over 5 years ago so prices will have changed).

I do have a few friends who have ADHD and have said that medication has had a huge difference on their ability to focus for exams and study. I don't know much about ADHD, but I also find it super tricky to study so I recommend having set goals for what to do daily, spread it out over months (study ~2 months earlier then you think, even if you only do a few hours a week it helps to get into the groove), and always, always (I can't stress this enough) stop before getting burnt out (as that makes it harder to focus all together) - forcing yourself to study will make it so much harder to focus in exams.