I have lots of opinions so I tried not to put them in my OP as I tend towards very long posts...
Lots of things that have been suggested on here (like detentions for wrong socks, not having a pen, not being allowed to the toilet in lessons) don't apply to my school and yet the kids don't seem to be skipping happily in so while I think super strict behaviour policies contribute, they're not the only explanation.
My school isn't a nice place to be, tbh. The building is crappy, should have been torn down and rebuilt years ago. Way too hot in the summer, too cold in the winter, dirty, faded and generally depressing. The curriculum certainly feels overcrowded, we don't seem to have room in the year for fun projects or stuff that doesn't directly contribute to improving test-passing knowledge anymore.
There's a severe shortage of teaching staff and kids are having lots of cover lessons where basically nothing is learned and behaviour is terrible. Everything feels rushed - breaktime is nominally 20 minutes but by the time they're outside there's 15 minutes left, then five minutes before the end they are herded to their next lesson so effectively they've only got 10 minutes to chill out. Lunch is now half the length it was when I started teaching.
And teachers are so stressed - that also has to have an impact.
Parents always moan about schools doing 'fun stuff' in the week before Christmas as it's a waste of time, so we don't do that anymore, we teach right up till the end. I don't think that's necessarily a good thing.