Yes, ADHD is definitely an extreme version of things most people experience. It's over-diagnosed, and it's sad that people will use drugs as first resort, instead of teaching better organization. But it does exist; by definition, anything that hampers your life can be diagnosed, and severe ADHD does.
If you cannot focus when you want to focus, there's a problem. There's a difference between needing some time for unstructured play and actually being unable to focus when you want. People with ADHD generally have trouble focusing even on leisure activities--it's not just schoolwork or "boring" things; they'll forget they were supposed to go hang out with friends, lose track of what they were doing in their video games, lose their possessions so repeatedly that they've replaced their driver's licenses or keys a dozen times in the last two months... This is a problem. Kids do need to run; and in fact ADHD kids learn better when allowed to move while doing it. But ADHD is real; it's not just annoyed teachers trying to drug a rambunctious kid.
Best approach for ADHD? It's not drugs and it's not beating the kid into submission; it's education. You teach somebody to create external structure so it's easier to focus. Kids with ADHD should be taught things like using to-do lists, creating routines (like putting your keys on the same hook every time you come home), and finding out how they learn best and taking advantage of that. Maybe you use stimulants to help them focus while you teach them; maybe you don't. Maybe you just give them coffee. Maybe, in the worst cases, they stay on stimulants even after they learn; but in any case, the stimulants aren't there as a cure-all; they're there to make it easier to learn.
If a child is "drugged" or lethargic at all, the dose needs to be lowered or the medication needs to be stopped. People don't learn properly when they're half-asleep. I don't care how much trouble a fidgety child causes for a teacher; there is never an excuse to use drugs or restraints to make him stay in his chair.
I'm a college student now. I fidget while I learn--deliberately. I rock back and forth and play with things in my hands and tug on my hair and pop up and pace around. I wiggle my fingers and tap my feet and hum and whistle and stare at shiny things. It's how I learn best. And yes, I have ADHD. Oh, and either autism or Asperger's, depending on which doctor you ask. Honestly, there's really nothing horrible about either of those things, despite that many educational systems would be glad to put me on Risperdal (nasty stuff) and pin me down whenever I got too much to "handle".
What the heck is so wrong with being different--even being disabled? Nothing. Not to me. Apparently, to the people who like to label others defective, there's everything wrong with it and we've all got to be "fixed" like we're broken appliances.
I have a weird brain. I don't learn the usual way. That shouldn't be an excuse to try to drug me into a stupor. Any dose of Ritalin more than 5 mg turns me into an instant zombie. (More proof that ADHD is real: Most people don't turn into zombies on stimulants. They speed up.)
Mercury=either autism or ADHD? No. It sounds nice, but it's pseudoscience. You're born with a strong susceptibility to ADHD or autism, and you pass it on to your children in your genes. Environment can make it worse; but usually "environment" is things like being born premature or having a mother who was on drugs, not the mild mercury exposure the average kid gets. We do actually have kidneys to handle toxins like that, y'know.
It's just that recently they have turned mild ADHD into a disorder, re-named "minimal brain dysfunction" as severe ADHD, and differentiated autism from mental retardation (nice for the autistic people who are not mentally retarded--that's most of us--and can now get a decent education... theoretically). The number of kids needing special ed has increased only slightly, probably because we are better at keeping disabled kids alive--such as Down Syndrome kids whose lives are saved by surgical correction of heart defects. We're really just re-categorizing people. It still doesn't matter if you're retarded or autistic, though; it's still apparently a valid excuse to treat you like crap.
...wow, that was tl;dr. I rant a lot. Sorry. I guess if you read it all you probably don't have ADHD