I‘m a former primary teacher, current school governor and have a 5-year-old DD in Reception. I agree with many of the reasons cited by PPs above - things like excessive screen time/social media, lack of firm boundaries and Covid all play into the narrative of increasingly poor behaviour. But I think the full picture is extremely nuanced and much more complicated than straightforward ‘poor parenting’.
When I was teaching a few years ago there was always a sizeable group of challenging children who needed additional support with behaviour, communication etc. This group was largely made up of those who were socially or economically vulnerable, and whose parents themselves lacked the education or resources to parent effectively - your classic ‘deprived kids’.
What I see around me now, though, - in the school at which I’m a governor, my own child’s school year and my peer/friendship group of mums - is that this group has been joined by another group of challenging children, who are materially much better off and more ‘middle class’, but whose issues with behaviour etc seem to stem from lack of parental time/input. They often have two full-time working parents who are run ragged trying to juggle everything, have been in full-time nursery from a very young age, spend much of their time outside actual school hours in a ever-changing mish-mash of breakfast clubs, after school clubs and holiday clubs, and are given no boundaries in the time they do spend with their parents, because said parents are exhausted, suffering from parental guilt and are desperate to spend ‘quality time’ with them. It’s really noticeable that all of the most challenging children in DD’s school fall into one of these two categories.
This is by no means a criticism of working mothers/parents; most of my own friends have no choice but to work full-time, and I’m aware that many people manage to parent perfectly well while also holding down a demanding job. But something seems to be going on whereby a growing tranche of middle-class society is now so chronically stressed, exhausted and time poor that they are unable to parent effectively, and prepare their children for school and to participate in wider society.