That's true. I think the other places where it happens often have similar dynamics to schools. For example, in a workplace you might not get to choose who you work with, or be able to leave easily. That's less true of other cases like bullying in teams or clubs though.

I just know that, personally, most of the times I experienced bullying in school happened when I was separated from my friends. I think that the natural tendency people have to look out for each other is stronger than any tendency towards bullying, but that schools disrupt the balance.