You sound exactly like me when I was 15. Luckily, I was already seeing a psychologist for other emotional issues who was very open to exploring the possibility of having ADHD with me. From what you are describing, it sounds like ADD-inattentive type. Is there is a learning specialist or a counselor at your school? I would start by setting up a meeting with them and explaining your issues as they pertain to school. If they think it's worthwhile to test you for ADD, then they could help facilitate that conversation with your parents. They could also probably do the testing at school for free (if you go to a public school) to see if you qualify for any services or accommodations.

A side note- I don't think "confronting" your parents is the right way to go about telling them, if you decide to tell them. I think explaining to them on a premise that you are having trouble at school and explaining the symptoms you are experiencing and asking them if it would be possible for you to be tested is a better way to go about it. I imagine they want you to succeed at school. If a diagnosis will help you understand how to better learn the material and manage your symptoms, I can't see any objection in them allowing you to get tested. The worst that can happen is that the test will come back negative.