You say you have "known" you have ADHD for a while, the question is have you been diagnosed with ADHD? if you have not been properly diagnosed with ADHD then there is no reason to tell anyone, except your parents, that you think you may have it. And even if you do there is no reason that you should have to tell your coach as it is a private matter and really none of his business. And if you did tell him why should he or anyone treat you any different? Finding out you have ADHD should be a weight off your shoulders, now you know the reason why you have difficulties in some area's. If there is anyone you should tell it would be your school nurse and only if you are taking medication. Having ADHD doesn't make you less of a person, although ADHD has it's negative effects, and they get worse the longer one goes undiagnosed, it also has many positive effects. People with ADHD/ADD are not stupid in fact most have very high IQ's. What we do is our brain processes information differently, many times we want to do one thing but our brain prevents us from doing the way we want and when we want. Medication helps with the focus and slows our racing mind down but we also have to force ourselves to stay on the task at hand. I recently read a comment from a doctor ( who is well known and has ADHD) who was explaining to a young patient, who he had just diagnosed with ADHD, what it was. He said " A person with ADHD has the brain of a Ferrari, very powerful but with brakes from a bicycle. The negative stigma of having ADHD usually comes from people who know nothing about it and if they claim they do its usually very minimal information that was passed on to them from someone else who doesn't have it. The key to dealing with it successfully is educating yourself, the resources are abundant. Your school may have a policy about athletes filling out a medical form prior to playing sports, if you are taking medication for ADD you should let your schools nurse know. They are the ones that clear students to play sports and usually they are the only ones that know whats on your medical form. Your coach has know reason to know and your not obligated to tell him.