You can't insist she takes medication. Billions of people with ADHD survived without meds until very recently. If she needs a break from it, let her. And trust that she knows how her own body reacts. f the pill stabilised her moods, then perhaps she should go back on it. When she's open to discussing medication you could point out that there are several types - slow acting, fast acting,stimulants or non-stimulants, so it might be worth trying a new type, if she wants to.
What will help her, enormously, is to do some organising for her, as she can't. Not won't, can't, so it doesn't help to leave her to it, much as you might want to.
To help with school in her final year, get the timetable for classes and for homework printed out large and stuck on the fridge or somewhere easily visible to you both. Colour code each subject, and put big colour-coded stickers on everything she needs for each subject. Put all items for each subject into a colour-coded ziplock bag. Double up on pencil cases etc if you need to, so she always has everything she needs for each subject. We did this for DS (AuDHD) and he went from being the least organised person at school, to the one who always had what he needed for each lesson.
While she is off medication, try to help via her diet - oily fish, lean protein, walnuts, berries, complex carbs, magnesium supplements all make a small difference.