When kids realize that there are consequences for bullying, and that they have to stop, they will. (The above goes for physical abuse, mostly. The solution for verbal abuse is "learn to insult them better". They either learn to stop insulting you, or they fall back to physical abuse, in which case, see above.)
That's what my parents did when I was bullied in middle school. They called the other kid's parents, and he stopped. There was one time they did need to threaten the principal with calling the police, and she pleaded with them not do it, promising that the kid was getting psychological help for his issues, and they agreed to conditionally back off (and he never threatened me again).
Bullying is already socially unacceptable -- it's against the law of society. We can solve it the same way we solve every other criminal activity, which is by involving the law enforcement authority.
Edit: Changed "socially acceptable" in the last paragraph to say "socially unacceptable" (was a typo).
If a kid's parents do not care about him, I'd like to suggest that the least of that kid's problems is getting bullied in school.
Truer words have never been spoken. Fighting isn't a first option--it's a last option. But if you were teased in school (like I was), the last thing a bully expects is a kid that will rage on him.
Making bullying "unacceptable" isn't a reality.
Children need a justice system that they can respect, which will only happen if the system respects them back -- treats them like first class citizens with a voice and control over their world. The adult system can't do that, so they probably need one of their own. As their educators, adults need to help them build it.
Children want civility as much as adults do, but as long as they have no control over their world, they will not feel any responsibility for it.
Children aren't mini adults. They're half done adults. There's some scary and important stuff missing.
Not being little monsters should be a part of their education. Instead, they have to grow up and live in the adult world for several years before they actually learn to act like adults.
Well, you're wrong. I don't want to get all Heinlein on you. But this is the way of the world. If you're cold, you put on a jumper or turn up the heating. If you're hot, you have cold drink or you sit in the shade. If you don't do one of these, the situation remains the same. And if you're threatened, you fight or you run. If you can't run, you fight. And if you have to fight - trust me on this - you have to go all in.
The smart solution to bullying is to make it socially unacceptable
If you have a way for an 8-year-old fat kid to do that, let's hear it. Hell if you have a way for "society" to do that, let's hear it.