It's constant stimuli. There's constant traffic noise, flights going overhead, phones, classrooms filled with masses of displays for the benefit of parents and Ofsted, not the kids. Hardly anywhere is ever properly dark or properly quiet. TVs are on all night (and even if you don't have yours on, the people next door will have theirs going). There's an expectation to leave lights on in their room, landing lights on in case they need the toilet, so any stirring leads to them waking up fully, their sleep gets disturbed.
And then they have to perform from far younger. It never used to make the slightest bit of difference if a kid took a bit longer to read fluently, as they would catch up by the time they were 10/11 for the 11+ or if not, there would the exams at 16 - now, if they haven't improved a set number of sublevels in 38 weeks, the teachers get hauled over the coals for it.
Activities that weren't solely achievement based were usually free or very low cost - now Music lessons are expensive and parents expect children to go through the exams every year to see they get their money's worth, rather than 'he enjoys it', sports clubs cost loads instead of being offered free by the school because the school can't afford it. It's vital for kids to get into the best most MC school and will travel for hours each day in the process, rather than the vast majority of all abilities going to the nearest.
It's harder to get decent food - there's loads of crap punted out to them on the grounds that children need sugar cheap carbohydrates, even in schools.
The constant noise - visual and auditory - and neverending demands on them to be better than average means they don't get chances to just be. Kids who would have been in special schools are now in mainstream because there's nowhere to put them and it's 'better' for them to suffer there than have them in smaller, safe and secure environments - a small primary won't be able to survive financially, their results won't be good enough and they'll be closed or swallowed up by Academy Trusts. To make money to pay for the leaking roof or a rebuild, they have to academise, sell off the field where you used to be able to go and sit at lunchtime for housing and you don't get the 'it's a nice day today, we're going to have our lesson outside', because it doesn't fit the Scheme of Work and Lesson Plans that were set and given to parents in September (if not earlier).
Teachers don't - can't - stay in one place for many years, as they become too expensive, the grades are constantly scrutinised and they have to keep on moving every couple of years, so there isn't the stability of school staff being the same. The NQT doesn't stay there and through the years, work up to becoming the Head. It's seen as a negative thing to have been somewhere for years on applications - being a constant presence and security when a child's life could have been filled with anything but security and reliability is seen as a bad thing on an application.
There were still MH problems 40 years ago. There were still suicides, kids disappearing from one school without warning, still depression, but we know about them now instead. We were taught meditation in 1982 aged 9-11 because teachers were trying to combat stress and anxiety in children. Now, if it ain't in the National Curriculum, it ain't happening.
It's a lack of downtime. A lack of rest. A lack of filling food, vitamin D, Omega 3 fats, a broad diet of seasonal produce. A lack of play.