Hey!
Thanks for your post. This is a very good question.
I have a student with severe ADHD and it is extremely challenging.
I have spoken to some family therapists and have done some research and let me give you some tips on how I deal with this kind of student.
1. Don't over explain things. Be brief and concise.
2. Don't ask open ended questions. They have a tendency to drift away when faced with deep intellectual questioning.
3. You have to be a little firmer with them. They constantly test you.
4. Their world is very black and white.. this is not their fault. Do not hold it against them if they are occaisionally brutally frank with you. You're not going to be able to change it. If you want to avoid this, refer to rule number 2

5. Give them something else to do. You have to realize that kids with Attention problems will actually get worse if you force them to concentrate on one thing. This seems counter-intuative but giving them something else to concentrate on while you teach them is actually helpful.
- Examples. Give them silly putty to play with or a strees ball to squeeze... Ever notice how they are constantly touching things and looking around?
6. Have a lesson plan prior to teaching them.. don't wing it! You'll need a strategy, their lessons need to be very structured.
7. Don't just have them sit at the piano. Move them around the room. Have them practice standing up, playing games with them at the table, on the floor etc..
8. Keep the lesson moving. Keep your topics short. Don't spend 25 minutes explaining their piece to them.. they can't concentrate that long. Switch things up on them a lot. Theory, games, technique, repertoire... short intervals.
9. Stay calm... If you get rev'd up, they will get rev'd up and then you'll lose them.
10. Keep the parents involved. These kids need a lot of support in the studio, but most of all, in their own homes.. if their parents aren't willing to work with you, better pack up and go home.
I really hope this helps you out.
Best of luck to you!