Well, here's my take.
Socialization is the process by which your children learn to get along with others.
One theory is to throw your kids to the sharks and let them figure it out. That will include bullying, humiliation, pressure and so on - because kids at school are indeed left to sort it out by themselves, with very little adult guidance. Unfortunately, kids who are bullied don't get stronger for it. They develop protective mechanisms (like withdrawing), just like they would with any abuse. Children who are abused by their parents are not better off as adults, not better socialized, than those who are supported by their parents. Same goes with kids who have been bullied, ostracized, etc. - they aren't healthier, better-adjusted adults for it.
Yup, you might encounter bullying as an adult - though it's probably a lot more rare than in school. What would somebody do who encountered a bully for the first time as an adult, with an intact self-esteem and no prior experience? Well, the details might vary, but a healthy adult who hasn't suffered years of bullying is probably better equipped to say "take a hike" to the would-be bully than one who was bullied for years. Think about it.
I know I'm focusing on bullies and that's the extreme end of "socialization" in schools, but I think it's just easier to see the constrast with more extreme examples. Personally, I was not bullied in school, but certainly ostracized. I am not more social for it, trust me. My mother says I was a very friendly child but as I grew older, I became more aloof and suspicious of people. Yes, I have to deal with people. I have to deal with people who might reject me even as an adult. I do think that if I was not thrown to the sharks, I could shrug that off as an adult better. Instead, potential rejection as an adult just brings up all those old wounds.
So I guess I'd ask your DH to explain what he thinks the process of socialization is. Does he think a kid facing non-compassion from other kids will emerge with an open heart and a set of excellent skills to deal with it? Looking back on his school time, can he think of some kids who got picked on a lot - and does he think those kids are, as adults, friendlier and happier as a result?
Here's another thought. A kid dealing with humiliation or ostracizing or whatever in their formative years has no choice but to just suffer it. An adult dealing with humiliation at work has choices. I can almost hear your husband saying "well, that's the thing, you can't just up and quit your job because somebody doesn't like you." Of course not, but who is more likely to brush off someone who is not toxic but just unfriendly or annoying or critical of you - someone who has dealt with that over and over and over again throughout school, or someone whose self-esteem is intact? Furthermore, what is the ideal reaction when dealing with someone at work who IS toxic - abusive, etc. To just accept it? Or to say "no, this isn't ok, I'm going to (tell the guy to back off, talk to the supervisor, request a transfer, whatever)"
In the adult world, we do encounter people we don't like or who don't like us, yeah. Everyone I've talked to about this (and I have talked about this) agrees that the adult world is so much better than school, though. You have so many more choices. And there just plain are fewer bullies in the adult world.
Sorry for the very lengthy and totally unorganized answer. I might not have evern really answered your question. You've just touched upon some feelings and experiences I've had. My DH also feels he is less friendly, less open, more suspicious, more sensitive to judgement and so on based on his experiences with the hellish social structure of school, of which he was even sometimes at the top of the pack.