There are parents who basically enable their DC.
There are the DC who run riot at home, and when they get to school they have to spend two years learning how to sit still for more than five minutes, disrupting the class in the meantime. These children have no SEN, they are just poorly disciplined and unfamiliar with having boundaries imposed on their behaviour. They settle down by about Y2/3.
There are DC who come to school and refuse to do any work except what appeals. They play their parents off against the teachers, their parents always take their side (always believe the child rather than, say, the two TAs and several children who give much the same story for a playground incident) and seek a diagnosis for the child. These children do not have SEN or MH issues. They are able to manipulate their parents who never impose any boundaries. Schools won't get heavy because they are scared of the parents complaining, and dealing with an official complaint is even more hassle than placating the parent.
Then there are parents who actively damage their children. I don't want to go into detail about one case that affected a family where I used to work, but it was pretty much Munchausen's by proxy: the mother wanted her kids to all be autistic or have some form of SEN. She screwed them royally along the way.
There is a significant cohort of parents who just don't take any serious responsibility for their DC. They seem to think that ALL their educational needs - politeness, reading, table manners, maths, conflict resolution - are entirely the responsibility of school. They send them in with no breakfast, despite having gel nails and new phones - money is tight in a lot of these families, but it's not well-managed. I've been poor, we were very poor during my teenaged years, so I do get it, but there is poverty and there is borderline neglect.
Then you get the parents who HAVe to go to the gym every evening, rather than hearing little Elsie read, when she is a year behind her cohort and doesn't think reading matters, because her parents have never shown her that it does.
How we deal with this, I don't know.
<and breathe>
Also, hats off to the many excellent parents out there. the ones who pursue diagnoses for DC who have genuine issues and get the EHCPs young enough for it to make a massive difference. Who support the school and come up to staff outside school and say, Thank you so much for all the help you gave him, it changed his experience of school so much.