I live in a pretty small town not far from NYC, with ~1500 kids for all K-12. Many of them would stick around for all 13 years, so pretty much everyone knows each other.
I could be wrong, but bullying seemed to be pretty uncommon for my year. I'd say maybe only ~5% of the kids were "weird" enough to be picked on. In middle school, those kids would be terrorized by my friends, so much so that one moved out to get away. My involvement was mainly just laughing at what they were doing on the side (yeah, I'm still guilty).
Of course, people grew up, and I didn't see any happen in HS. The main offenders apologized years later, via facebook. Nowadays I'd like to think I'd stick up for the victim if I were in that position again. Regardless, even now we still joke about some of the horrible things we did (yeah, we're fucking terrible people).
Still, I have a hard time believing bullying is that widespread - when I see it in movies, it puzzles me. It seems the reasoning Dodece put up for why bullies mess with people is way different than my friends' - for them, it was more for the laughs. For whatever reason, we were complete assholes when we were in a group, and generally just enjoyed screwing things up. There was never any "solo" bullying, because that'd defeat the purpose. It was also more tame than examples I see, being entirely psychological (teasing, pranks and such). It may be just as bad, but I guess as kids you don't realize how damaging it still is.
It's an interesting subject, though. I'd love to know how other schools behave in different areas / cultures... Recently I've been talking to someone that tells me she were bullied, and I always thought she was in the minority. I guess my school might be the odd one out, though...