I tend to agree with this to a large extent, although the parental lack of discipline in homes also feeds into it. I think it would be possible theoretically to manage discipline in a way that was effective.
But it's not just about excluding really violent kids. It's about expecting an orderly, well behaved class and taking appropriate measures to correct it even when the bad behavior doesn't rise to stabbing or throwing things or whatever. You will never get self-regulating kids if you aren't starting with the small stuff, at the beginning. That's where it is simple, baby steps, once you get to extreme beahviors it's going to be difficult for kids to make the leap.
But the way our culture thinks about behavior and children now, that's not how most people think about discipline. Right down to toddlers and even younger, (and weirdly this is also true with animal training) the philosophy seems to be that you will get good behavior if you love kids and give lots of positive reinforcement. There is no need to directly try and extinguish bad behaviors, and in fact doing so is thought to traumatize children.
Of course if you don't do this what you end up with is children with little capacity for self-regulation.
It's true schools aren't really allowed to do this effectively, even where there is a will, but many school managers and even teachers are influenced by this dominant cultural message. They, like many parents, may struggle to be consistently holding kids to higher expectations of behavior, because it seems wrong. And we no longer know intuitively what it looks like either, which is why when we go outside our culture to a place like France and see kids who have an overall higher level of self-regulation, it seems weird and magical.
Just as a note, this parenting approach now seems to dominate throughout the English-speaking west, and they are seeing similar issues in schools. It's not just a UK thing, so I would say while issues like staffing exacerbate it, that's not the main cause. The real thing to explore is this cultural attitude around teaching kids, behavior expectations, how people think kids learn, and so on. There are some very odd underlying assumptions around children having a kind of naturalistic goodness that will emerge so long as you let it do so freely. And very little understanding about how we learn good habits, and that these are actually very freeing and empowering once you have them.