grade 12 can be stressful because you have a lot of serious decisions to make with very little experience in how to make adult choices, so hang in there. Teens with ADHD typically have the following sorts of problems: controlling fidgeting, sustaining attention to boring tasks, completing and turning in work, time management and loosing their stuff. Young women with ADHD often primarily complain of trouble paying attention, though they can also have problems with impulse control (giving in to temptation without thinking--such as shoplifting, sex, cutting class or smoking). ADHD does not spring up 'out of nowhere', so if you are having your symptoms all of a sudden, it is less likely to be ADHD and more likely to be situational stress. Only an assessment will tell you for sure.
Because you are under 18 (I assume) and do not have your own health insurance, you will need to get Dad on board. I would meet with your guidance counselor or school psychologist and describe your problems. Ask that person to serve as a 'go between' to help your father understand what you are going through and what kind of help you need. You may also be able to find a coach, relative, or clergy member who could do this for you if the school staff are not helpful. Keep trying until you get someone who is willing to listen! I am sure your father does not love the idea of you not graduating and working fast food for the rest of your life, so he may come around given time.