If you go to a doctor then they can't legally keep anything confidential form your guardians until you are no longer a minor as far as I know. Why are you afraid of your parents finding out you have ADHD? It certainly is something your doctor might be able to help with as you may be able to get some medication for it that can help you improve your feelings and behaviour. There's nothing to be ashamed of in having a disorder of any kind - why would your parents be unhappy with you?
There are other things that can help ADHD. You say you're already on a behaviour course at school? You might be able to get a better course through your doctor with a trained counsellor that could help you. Also there are dietary changes that can really make a difference when it comes to ADHD - such as removing as many artificial additives as you can.
This really is something you should discuss with your doctor because I am sure that he can provide you with more comprehensive help than your school alone can.
Are you worried because your behaviour at school has caused you to get into trouble - hence the course you are on? And you don't want your parents to find out?
If that's the case then what about getting a teacher to talk to them first and explain that you aren't in trouble but you have a condition that you need to see the doctor about? I'm sure if you got a teacher who understands to explain it to them for you then they would have to take more notice and understand that you need help.
Please try and do that and try to have the courage to let your parents know what is going on so that you can get the help your doctor can undoubtedly provide - you have a lot to gain here and I think it really is worth it.