“Inherently” is its own reason. I don’t think children and teens are inherently cruel, I think they are cruel for reasons other than “it’s their nature”.
I agree with this and disagree with the folks saying either that human nature is just like that or that children haven’t developed morally enough yet.
There’s a term for what jtur88 alludes to: horizontal oppression. I was a children’s libber when I was a child. The oppression of children is complex and different from a simple black & white analysis of power, as it involves care and protection and it’s obviously oversimplistic to view it in terms of evil mean grownups exploiting children for their own advantage. But children are kept artificially disempowered and treated as lesser entities in ways that they experience as a social inequality.
And we resent it, even if we don’t always analyze it in terms of “children’s rights” so much as “fucking teachers and my damn mom and dad” etc etc. And that anger and resentment bursts out sideways, encouraging coercion and bullying and tormenting and belittling (study that word, please) of other kids.
It reaches its apex when children are the most aware of it yet not as of yet receiving many of the benefits of being perceived as “almost adults”: middle school, or what we used to call junior high. (& yeah, the tensions of new hormones sure don’t help).