Exercise increases the endorphins in the brain for a couple hours and some use it to help focus. However, doing something that you are not particularly interested in can be frustrating and stressful; ceating the opposite effect.
I think martial arts is an individual thing. You either like it or you do not. If you like it, it builds body confidence, discipline and coordiantion. I do not have teens I have an 11 year old (undiagnosed, suspected inattentive ADHD) that is a brown belt in kung fu and loves it; and a 10 year old (diagnosed combined type ADHD) that starts and stops all the time. She gets bored and takes a break, when it is time for testing for the next belt, she stops for a while and has to do everything over again. We have not restarted since she has started meds, however, so things might be different now.
Maturation alone can reduce or eliminate the need for meds, but that is also individual. Some need meds their whole life, some are fine once they get out of school. Good luck.
Our daughter becan karate at 11 because she would sit and watch her younger brother at class. She had no physical confidence and lacked co-ordination and muscle tone. I thought it was a phase so I let her join - 3 1/2 years later, she obtained her black belt. Since then she's dropped karate (numberof reasons!) which is a shame. She did learn how to set goals and work towards them. A little bit of a drama queen, she would cry and say she could do a certain move etc but the instructor whould break it down and she'd master it in time for grading.
It was great for her social skills and self esteem- she was forced to interact with others boys and girls and as she rose in the ranks, she would help younger students and also demonstrate moves and even take classes.
Find a martial arts school that has special classes for girls / teensagers. You may need to sit in on a few to find suitable classes. The instructor is so important!!!
My daughter is 8yrs old and we are considering a martial art for her. My hubby suggested Aikido as it is more a defensive art than offensive. (blocking blows using the opponents energy rather than kicking and punching etc) Finding a Dojo in our area is a problem.