ADHD, not severe but not mild either.
I'm currently on Concerta, and have been for a bit over 2 years, I tried adderall when I was 12, and refused to try anything else for another few years, cause Adderall sucked. I don't want to stay on meds for a really long time, but I suspect I'll have to until I finish high school, and perhaps college. It depends. I can focus reasonably well as long as I'm allowed to move around as much as I want and the subject is interesting. When I was little, even before I got diagnosed (I had to go through a long evaluation which also found very mild dyslexia before I was considered ADHD), I could never sit down for long, and me teachers hated it, because I wanted to walk around the room, and they wanted to teach math.
Now, when I forget to take my meds (which happens periodically), I drink coffee and I ask my teachers to understand that I may be a bit hyperactive and need more time on tests and stuff. I've actually taken whole tests while standing up instead of sitting, which my teachers are understanding about, and standing actually works fairly well. There are still issues with homework when I'm not on meds though.
Oh, and most ADHD meds are stimulants, same as caffeine. Something like coffee will cause similar effects to meds, although they are milder and less precise (a doctor who I see who specializes is ADHD actually told me that when I had been refusing to try any meds). So, for very mild ADHD or ADD, coffee would probably suffice, and for cases that don't involve homework it also works. Thus, my plan as an adult: become a coffee addict and ditch the meds ^.^