Parents are definitely my number one.
When I started teaching, the threat of phoning home often used to be enough to get a child to behave and you only had the odd one or two who'd misbehave. I would, maybe, need to actually make one call a week. Now I dread every email and every phone call about behaviour, because instead of supportive parents I now have parents having a go at me. And those phone calls have to happen daily, because behaviour is much worse. I expect other adults to be reasonable, and I think that is why it is so upsetting when that doesn't happen.
Behaviour from students has two sides, which are equally appalling.
One is a lack of resilience and taking responsibility. Classes where half the children don't have a pen, let alone a calculator, where maybe 2 out of 20 have done their homework, where "We have a test???" comes after numerous reminders in class, on their homework and on my slides. Where children will ask to go to the toilet 10mins after break, because "break is for chilling, not going to the toilet", and then walk out if you refuse. Kids who speak to you however they want, but if you say one thing that can be even slightly misinterpreted (or, you know, have sass yourself), you get hauled into the safeguarding office, because little Gracie felt upset.
The other is that students now feel entitled to completely ignore you. You greet kids at the door and are lucky if one in 5 acknowledges your "good morning". You ask them to tuck their shirt in in the hallway and they walk past as if you don't exist. They're talking in class, you stand next to them and talk to them to address it, they carry on their conversation as if you're invisible. I find that behaviour worse than being told to fuck off.
As for reasons? Schools buy into "When the adult changes, everything changes". Any misbehaviour that carries on or escalates is down to the teacher, not the child. We're being gaslit into believing that is is our lack of building relationships, our lack of yet more accommodations, that has led to the child kicking off. Children bear no responsibility for their actions or their grades and if you treat them the way the real world does, you're in the wrong. Thankfully, we're seeing a slow return to actual discipline in some schools. Which then also report rapidly improving behaviour.
And a definite yes to "I only need to pass Maths and English". When did that attitude become acceptable? If you teach anything other than Maths and English at GCSE, you're stuffed, but if you're an EBacc subject, you get told to emulate the excellent behaviour and attitudes as well as grades in both subjects, while getting none of the support, curriculum time or children's engagement, because they don't need your subject for the next step.
Lastly, pay and conditions. Teaching used to have great conditions, all of which have been hugely eroded by MATs now (reduced sick pay, mat pay, increased days worked etc). I don't think there is an LA-maintained school left in my area. Pay was never high, but provided a comfortable lifestyle once you passed the first few years and that is no longer the case. With all the public sector pay freezes since 2010, pay is now worth 1/3 less than it was. It's why I am struggling financially despite my middle leadership post with vast amounts of responsibility, and why I'm climbing the greasy pole even though I have little interest in senior leadership.