I've been working in schools for over a decade now.
When I was at school I went to a primary in one of the worst council estates in the country. We had about 2-3 kids in the class who might be termed "tricky" in their behaviour - I was one (I'm under no illusions - I was a little shite at school - very very intelligent and bored... that hideous recipe for causing utter mayhem), another was a lad who was just a bit mouthy, and another was a lad who had genuine massive issues. One day he lost it completely, snapped a ruler and walked out of the classroom - we were all utterly, utterly stunned by that.
Fastforward to me starting to work in schools and you'd tend to find about 5-6 of the "lively, little bit mouthy, need to keep on top of them" type kids in the class and the odd 1 or 2 dotted around the school of the ruler-snapping storming out level - but those acts had started to lose their shock value and the kids would just roll their eyes at the (much more frequent) kick-offs.
Fastforward a few more years and now pretty much every class you go in in lots and lots of schools has at least 1 or 2 kids who will kick off on a daily basis, storming out of class now merits no sanctions at all (possibly a chat with and a sticker from a learning mentor) and the "lively and needing managing" kids get away with so much more (and are so much more in number) because so much time and attention goes into cajoling the kids with the real issues into some form of compliance.
I've gone into schools on supply and been told "If X storms out - let him go, enjoy the break and someone will round him up en route out of the school" and another one where it's "this class is crazy gang with behaviour -just survive the hour and yes, they ARE always like that and it's not just you" (the second part of that was nice to know) and it just seems to be getting worse. In the metaphorical (NOT LITERAL BEFORE ANYONE GETS UP IN ARMS) carrot and stick situation - there is no stick, there's only carrots, and they've become so devalued they're meaningless - so you have kids expecting their sticker for each lesson and smashing up the classroom if they don't get one - so it's not a reward anymore - it's just an expected end of lesson routine.
What breaks my heart are the pained looks on the "always children" (the ones who are always sensible, always trying their best, always doing the right thing)'s faces when the room's in uproar again and they just roll their eyes and say "he's always like this Miss."
Incidentally - the reason I quit full-time teaching and will never ever go back is because I was begging through all possible avenues open to me for support for a child who had massive anger and behavioural issues - no one was interested in offering any support (SENCO, GP, Head, Deputy, KS leader... anyone) for this kid who regularly assaulted staff, smashed up the room on a daily basis, would storm out of class (oh how many lessons I taught from the classroom door keeping an eye on him kicking the corridor wall), destroy other children's work - and I made myself ill to a point I'll never fully recover from trying desperately to help him and protect the other kids' safety and education from it all. That child is the sort who, as well as everything else, is incredibly easily led - and I can quite see how his future is likely to pan out - he'll get peer pressured into carrying a knife or something stupid, someone will look at him and aggravate him... bam. It's terrifying to think of - but no one gave a stuff when he was 8 - will they give a stuff when he's 18, six foot tall and REALLY loses it? Because so much time and energy was devoted to pre-empting the potentially massive explosions there (you could defuse some of them by suddenly changing the pace of the lesson, or moving from desks to carpet or similar) - lots and lots of low-level stuff then starts to creep in and make things even worse (and there are always some who'll take the lend of a gap in your armour).
I suppose if Ofsted had rolled in when I had that class they'd have blamed me for what was going on in there - but I can put my hand on my heart and know that I did everything within my power and gave everything I had (and paid the price myself in terms of the destruction it did to my health and wellbeing) - and I don't think, without the backup from higher up, anyone would have managed that class better (indeed when I went off-sick the supply refused to teach the child in question, the person who took the class after me also left within a year as well). I think with lots of these cases the blame for a culture of disruption lies much further up the line than with the classroom teacher - it's up into the hands of school management - if there's no behaviour system in place that's working, the buck stops there.