I watched my kids becoming a median of their classmates. After changing to a private school it was noticeable after a month - different lexicon, mimics, attitude.
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.
And I grew up among median kids till I was 19 and never did adjust to the median (neither did my group of friends foinf to the same school). My younger brother however did.
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.
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.