The fact that it's natural means nothing. It's natural for small groups of humans to make war on their neighbors, to kill their men, rape their women, and enslave their children. But we don't put up with that kind of thing now, for the most part.
I agree with your last sentence, but my question is: why aren't they working hard to prevent it? Of course children have to learn this stuff, it's clearly not innate, but if they're going through their school years being bullied then clearly they're not learning the lesson.
Now, no doubt their responsibility is different, because their brains are different. It'd be appaling to jail a kid on charges of assault for a fight in school ofc.
However, this def does not mean that getting bullied wouldn't have similarly negative effects on the kid, as it would on the adults (easily worse, given the kid's personality is only forming).
Hence, the op was suggesting, rationally, preventing it should be just as high of a priority to the school, as it would be to the judicial apparatus in the case of adult assault.
Instead, it is just normal. And that is appaling.
Humans are naturally physical animals. Get some toddlers together, and (in addition to lots of other nice behavior like sharing and talking), they will push, grab, hit, etc. each other. We're primates and we do primate-y things.
As we get older, we learn the mores of our culture and learn to not physically interact with people in this way, but that process takes a lot of time. Kids haven't completed it yet (some adults never do). In the meantime, we have to provide an environment that handles the fact that they are behaviorally different from respectable, trustworthy adults.
Above, you say "We create these Lord of the Flies type situations for children", but the whole point of Lord of the Flies is that savagery is how children naturally act when taken outside of the strictures of society.
A Lord of Flies situation doesn't have to be manufactured by adults—it's what you get if you don't work hard to prevent it.