It's not uncommon for adolescents to be diagnosed. Especially for kids with high intelligence levels, it can be easy for them to slip under the radar and get by in school simply because they are quick learners and often don't display some of the behavioral problems that can come with the hyperactivity aspect of ADHD. And in contrary to what some believe, people don't outgrow ADHD/ADD. So it's not uncommon for adults to be diagnosed either. However, I think the process of getting diagnoses is many times more extensive when you are older, particularly if you are self-reporting attention problems. This is due to the unfortunate abuse of stimulant medications.

Do you have a school psychologist or a learning specialist at your high school? They would probably be your first point of contact if you wanted to try to get a diagnosis. They would be able to help advocate for you, especially when it comes to ADHD/ADD interfering with your school work. If they can't diagnose themselves, they could at least point you to someone who could. An evaluation usually consists of meeting with the doctor/diagnostician, taking some tests (how thorough the testing is depends on the doctor - mine was seven hours of testing), and basically reviewing your history of academic performance and any other areas of life that you feel are being impacted by your inability to focus. The course of treatment can vary as well and that will be up to you and your doctor to work out.