They've never experienced assholes, or people who think their personality is grating, or whatever. Thick skin needs to be built up, to a degree. I'm not saying bullying is good, but being exposed to the unwashed masses definitely can be.
It's not thick skin they are developing around assholes, it's them incorporating that behavior to some degree. There was nothing I could do to change that - talks, good example, nothing worked, only surrounding change.
There are kids who will have the tendency to adjust to others and there are kids who don't. This has to do with the self-confidence they bring with them before they enter their first day of school, friends they have/had outside school and it is not necessarily a thing a parent can always control in a predictable manner.
For all we know in your case it could even be that your kid learned they have to follow the others in private school and then applied that lesson in the next one. What I am saying here is that having learned the lesson "you need to adjust to your peers" is the problem, not that the peers are the "wrong" ones.
Given the state of our education system, that seems like an obviously worthwhile trade off. Especially for parents in underperforming school districts and even entire regions.
The same applies to teaching "street smarts" to kids. You don't do it by throwing them in a hostile environment where they'll be prey to hostile people without having any defenses built up first.