This is more complicated than perhaps you understand. You didn’t indicate how you were diagnosed with ADHD and how it is that your parents aren’t aware of it. If you were diagnosed by a professional, the next step is to get some treatment and to involve your parents in supporting it. But if you made a self-diagnosis, I think you may be incorrect about it.
It looks to me like your relationship with your parents has become very stressful and distressing to you. You seem to be caught between your desire to make your own life and your loyalty to your parents and their expectations. One not very useful way out of this dilemma is to blame a disorder and to feel ashamed. Then it’s not your fault when you don’t do well on exams. But it also means you aren’t moving forward and doing what you need to do academically to meet your own goals.
Even if you do have ADHD, it is not a reason to give up your goal. Many people with ADHD learn ways to make the symptoms of the disorder actually work for them. One person I know, for example, finds that the “distractability” that makes her move from task to task actually can be channeled so that she is an effective multil-tasker.
At 17, it’s time for you to be less focused on what your parents want and to take charge of working toward making a life for yourself. Your mother is right that it’s a good idea to keep some options open. But you can still focus on what you really want. Hopefully, you can have a clear and respectful conversation with your parents about what you want to do. You can prove to them that you are serious by doing what you need to do to be successful.
If ADHD is a factor, it’s important to share that information with your parents so they can support you in learning strategies for dealing with it. It is not going to go away. Most kids with ADHD do not “grow out of it”. Instead, they learn how to manage it. Please see a therapist who specializes in ADHD to get some practical help and support for self-management. If therapy is not an option, consider joining a forum here at PsychCentral. The forums provide a place for people who have similar issues to support and help each other.
I wish you well.