I'm new to teaching, so haven't seen the change as a teacher, but a long time ago in a county far far away I went to a fairly shit comp. I now teach in a comparable area/school I think.
I feel like it's respect for adults in general that has declined, the arguing back is probably what's shocked me the most (placement schools were good, I was unprepared). You give them a sanction and they argue, the other pupils join in, telling you 'Mr X lets us do that, Mrs Y lets us do this' (to be fair inconsistent enforcement of the behaviour system is one of the reasons this school is RI).
To be honest I'm not sure how much of this is them and how much is me being new (everyone had said their first year was bad, even people who came in experienced).
The entitlement - 'Miss, Miss, Miss!' Like they are the only one in the room. 'Are we doing a practical?' As they walk in the room. I would have barely managed 'Morning' to the teacher when I was their age.
The shouting out of utterly random things - think Ralph from the Simpson's but louder.
Waiting for quiet - fml - they start shushing each other, shouting at each other to be quiet, arguing about who was talking, sanction them and it's not fair cos z was talking too, sanction all of them (not actually all, there's a few that are actually quiet) and it becomes a game to see who can get detention first, then when they get detention it's unfair.
Of course, every time someone tries to observe this they're good because it's a teacher they know!
This is one year group though, the others are ok.
Although - what happens to them in the summer before y7? I've seen primary - the kids in y4 are more independent than my Y7s 'Miss, where should I draw that? See, I've stuck this in and I only have 3 lines left, and that table is too big?' - use the next page Timmy, use the next page.
Parental buy in seems pretty good where I am, but I think the school takes quite a hard 'our way or the highway' approach to them doing their detentions, etc.