Why do you think all students are like your son? What do you think happens to the students that don't respond well? what about students who aren't in set 1 or2?
This is literally the problem with the education system, assuming all students are the same!
I teach in the best school in my area and my behaviour management is excellent but the reality is that all comprehensive schools have children who don't care/ can't do the work.
I teach an options subject that is mixed ability so no sets for me! One of my gcse classes has 1/4 of students with a reading age of less than 10. The gcse paper has a reading age of 15. They can work as hard as they like and I will make sure that they do, but they won't do "well" according to gcse scores so mostly they act out/opt out.
My second subject is vocational, they've chosen it as they think it's easier. It's not. So I've got a group of all lower ability. I can assure you many if them act out if they can't do it. One of those students has a reading age of 5. Obviously I differentiate the work but he still has to read and answer exam questions and this subject has coursework too. But I think anyone would act out (as he does) if they're constantly failing at the first hurdle. A small group setting it 1:1 I
So Obviously there are children who can't do it well and these children often fail and there's nothing we can do for them in an exam. Although on the flip side there are bright students who just don't care. So yes there are disruptions in lessons! Imagine if your son came home and told you he'd been threatened to move down and you'd told him oh well it doesn't matter, GCSEs are not important. Not all parents are the same either!
Everyone I know teaching is going above and beyond every single day. But there's only so much we can do. Students are people with outside influences and personalities, not robots who do our bidding.
And just because your son is motivated and works hard (good for him btw, I'm not being sarcastic, truly sounds like he gets it, thanks to you) it doesn't mean a smaller class size wouldn't benefit him. And it also doesn't mean people never interrupt his lessons!
FYI I have taught for over 20 years in Hampshire and Surrey.