The socialization question: People get confused about this a lot. They think it means “making friends” and “learning to interact with other kids.” For some reason, these activities have in recent decades been referred to with the umbrella term “socializing.” Actually, socialization (slightly different word, you’ll notice) means learning the social mores of your culture. If you look at the work of Jean Liedloff and others who have observed traditional cultures it’s pretty easy to figure out that adults are expected to socialize children into the practices of the culture. How do they do this? Simply by being in proximity to the children and reinforcing their behavior: we do this, we do not do that, don’t do it that way.
A school environment is so different from what children have traditionally had as a cultural learning environment to be almost something from another planet. The child has proximity to one adult at a time, in a room of thirty other kids. Contrast with the tribal environment where there are easily as many adults as children and usually more (given the low forager birth rate). What we have here is a situation where children lack the reinforcement they need from the adult because the adult simply doesn’t have time. What’s left for socialization purposes? Other kids.
Now what sane society would expect its children to teach one another about how to properly behave within that society? By definition a person who knows how to behave properly in a culture is an adult. They are recognized as such by their people. But if the person is defined in the society as a child and obviously doesn’t know how to behave properly then they have no business trying to teach other kids how to behave either. Yet that’s what we have in the school environment.
If you needed to understand why children in “advanced civilizations” turn out to be so childlike as adults, there’s the beginning of your answer.
The social interaction question: This is different from socialization and simply involves spending a lot of time around others who share your culture, language, etc. Adults need this, and kids need this. This is the part that makes homeschooling problematic. We’re a social animal, as y’all know, and we thrive best around others of our kind, both in a species sense and in a cultural sense. Our kids do too. Different people interpret the same culture in different ways so the more perspectives a child gets on their culture, the better-rounded that child usually turns out. Not to mention all the psychological benefits otherwise, etc.
But I’m not convinced school is the answer to this. It is only a partial answer and here are a few problems with it.
It is an artificial pseudo-social environment where the kids largely run the show, the adults are isolated from one another and overwhelmed with responsibility for too many children during classroom hours, and arbitrary rules hold sway concerning when people may talk and when they must keep silent. It is impossible to have healthy social interaction under these circumstances.
While all the people attending a school share the common culture of being civilized, (a) each individual person attending that school comes from another kind of home-based and family-based culture that often is not shared with their teachers/students/classmates and (b) wow, what an unfortunate thing to have in common, the experience of being mentally enslaved for someone else’s benefit. Because, make no mistake–school is all about the enslavement of a mind. It springs up universally in civilized cultures and the most successful schools are the most degrading, the most demeaning, the most hierarchal and controlling and the least likely to leave children with the ability to think.
And yet… It is hard-wired into the human animal to want social contact, and this need is strongest in children. Yet civilization does its level best to keep us divided and apart from one another, leaving us only with its structures to participate in if we want social contact–and its structures pervert social contact and twist it to suit civilization’s purposes.
I really think the best environment for kids to have social contact with one another in civilization is a playground next to an unstructured park. That’s not a bad place for adults to hang out either. It is not an optimal solution because it comes nowhere near replicating the tribal environment, but it’s a start.
Impossible to say what to do in the OP’s case, though. His child’s mom just wants what she still thinks is the best for their child, or the best they can manage given the circumstances. I also must admit bias in that I think that absent abuse and neglect, the parent who risked the most to bring the child into the world ought to have the most say about the child’s upbringing, and I suspect that was a common attitude among wild humanity, especially given what we know about the prevalence of matrilineal culture before the advent of agriculture (or, as in the case of North American indigenous, pre-Contact). But there is no clean, neat answer here that is best for everyone. Mothers have the right to raise their kids; fathers have the right to raise their kids; kids have the right to know both parents; and both adults and kids have the right to a sane culture. But we can’t have the sane culture yet so that skews everything else.