I had the exact same problem when I was a kid. It took me hours to do my homework when I was little because my mind would just wander. I probably would've been diagnosed with ADD or some shit too, but that's bullshit cuz all that was wrong with me was that I had a powerful imagination and it liked to lead my brain off the straight and narrow path. My solution to this problem probably won't work out for you. Sometime around age 12 or 13 (I think), I got the brilliant idea to just not do my fucking homework, LOL! So, I didn't, all through middle and high school, no fucking homework ever. Now we know why a super genius like me barely fucking graduated, eh?
I never did solve the problem as a kid. I assume that I'm able to focus now because of a combination of things, starting with simple maturity. While I hate the ageist rhetoric about the teenage brain, it IS true that greater maturity lends a greater ability to stay focused on a particular task (usually). But there were other things that helped me and they surely would've helped even without me having to get older. A basic study of the martial arts helped me to appreciate mental discipline in a variety of ways. I'm no Bruce Lee and today I couldn't name you a single fighting style that I learned when I was young and I'd probably get my ass kicked if not for always being armed and having no problem with fighting dirty. Still, I did retain some of the appreciation for mental discipline. Another thing that helped me was the study of Magic (I refuse to spell it with a "k" on the end, like some fucking new age sissy). I don't neccesarily believe in "magic" the way many who're into the occult do, but the study and practice of it requires mental discipline and that is of great benefit to you.
Hmmm... What I would suggest is that you study some more esoteric things. Do some research on Transcendental Meditation, Lucid Dreaming, and Astral Projection to start with. Even if you think that's all horseshit, set that aside for the moment. The point is that to achieve these things requires a mental discipline that will help train your mind into gaining the kind of focus you're looking for. Plus, lucid dreaming is fucking fun! You can also attempt to learn skills like Empathy, Telepathy, and Telekinesis. It is unlikely that you will meet with any great success in doing this, but it's the LEARNING of these skills that is important, not the success in employing them!
So yeah, you ask me how to concentrate better, I tell you to meditate on it. I know, it's a "Master Yoda" kind of answer, but it really will help. There are basic mental control skills that you need to learn that these things can teach you. The ability to clear your mind of all thoughts, for example (head-shrinkers will tell you this isn't possible, which is why they're fucking idiots). Let me know how this shit works out for ya.
All My Hate,