I have this problem too. I can't sit still and focus when studying. Here are some suggestions that might work for you:
-study for short periods of time (say, 20-30 minutes) and take a break (10-15 minutes). If you study for too long, you will find your mind wandering and start to get bored.
-drinking coffee or tea helps
-alternate between different subjects. For example, you have 2 exams you need to study for. If you can study for 1 exam for 30 minutes, take a break, then study for another exam for 30 minutes, and keep alternating. I think people with adhd have hyperactive minds so they can't focus on one thing for too long before their mind wanders.
I hope this helps and good luck!