I really want to respond to this because my instant reaction to the first two paragraphs was to be flooded with feelings of upset and anger. (This is not an angry post).
It is very tiring to be the parent who has tried the reasonable, normal parenting techniques, which do not work, resort to something extreme in a moment of exhausted frustration and/or out of desperation for something to work, have this be the snapshot which is seen (often because it is more visible or memorable by virtue of being extreme) and/or be using more of a "strike while the iron is cool" approach so it looks like behaviour is being ignored or indulged in the moment, and have this immediately judged as "Oh right, yep, one of those parents".
I know this happens, it's happened to me on more than one occasion. There have been two instances I can recall where the person who was judgemental initially changed their tone entirely when they encountered my younger child, who responds so well to "normal" parenting I am forever surprised by him myself, or the other time when I explained that my older child attends a very demanding, academic school, and my middle child's teacher visibly readjusted her opinion immediately.
Because no, I'm not conceited enough to describe myself as brilliant, but I think I'm a good enough parent, I do think things through, I do know the basics of good parenting, I try to be reasonable, I don't generally go out for wild threats or anything like that. I actually agree that most of the time, "special" kids don't need "special" parenting, they benefit from the same principles as all children - more praise than criticism, a good relationship with parents/adults, good adult role models, clarity around communication of rules/boundaries, a consistent and calm approach from adults, unscary and predictable consequences, recognition of progress, and fair expectations according to their age/ability.
The difference is that most people aren't following good parenting principles to some gold standard degree, and most of the time that doesn't matter. "Normal" parents are mildly inconsistent almost all of the time, allow emotion to come into behaviour management e.g. in facial expression or tone of voice or amount of correction, use threat/fear (even if it's not extreme), communicate boundaries in an unclear way, notice bad behaviour more easily than good behaviour and don't always know what is an age appropriate expectation (and if your child does have some kind of issue then often a typical "age appropriate" expectation may be inappropriate for them anyway). Most of the time, for the vast majority of children, this doesn't matter - they can fill in the "gaps" in clarity and consistency because of social learning and logic, some developmentally appropriate behaviours will reduce on their own as a child grows, and because they are generally able to follow rules more of the time, the occasional mishandled bit of bad behaviour doesn't make much of an impact on their overall self-image or relationship with their parents/teachers or is fairly quickly repaired if it does.
OTOH in fact you kind of do need to work to higher standards if you have a child who finds good behaviour much more difficult to maintain. It's not that it's anything special, because the same things would help all children and most good parents probably do try to follow the aspects they are aware of. It's just that most parents don't need to be so careful about their behaviour management and it doesn't matter very much if they aren't doing it perfectly, as long as overall it's good enough. But also because if your children generally behave OK, people aren't looking askance at your parenting looking for faults.
Anyway, sorry because this has ended up very long. But I wanted to respond, despite a general policy of not responding to posts which make me feel angry or upset, (which I don't any more anyway) mainly because your third paragraph reminded me that I do feel there is a communication gap here where parents and teachers misunderstand each other. I definitely feel that there are things from the teacher's side which I'm not aware of. As far as I've got with this is: There are genuinely bad parents, not just normal parents who are being assumed to be lax. So teachers aren't always being unfairly judgemental in making these statements. The gap is that I have no idea from my side what the distribution is - I find it difficult to believe that "decent parent with a genuine concern and an unusually challenging child" is as rare as this post makes out, because the majority of SEN parents I encounter (mostly online) would fit this profile. OTOH, perhaps my perception is skewed because I'm encountering a self-selecting sample, whereas schools collect a much wider range of pupils and families. I genuinely don't know, and would be interested if you would be willing to share your experience.
The other gap is that I think I assumed that if my child had a diagnosis, that would mean that someone would know what to do to help him. I don't know why I made this assumption, since it didn't have that effect for my older child, but my older child only ever had behaviour issues with me, he has always done well at school. My middle child is very disruptive at school. My hope was that the diagnosis would make it easier to communicate, because the school would be able to say ah right, yes, ADHD, we know what to do with ADHD, or if they don't know what to do, they could look up some strategies for helping ADHD in school, and some of it would help. But that does not seem to be the case, which makes me wonder what even is the point of the diagnosis (ADHD is not exactly rare). And BTW, our school is making a lot of the same behaviour management "mistakes" that totally normal, reasonable parents do all the time. They are not following gold standards of behaviour management, including things which I do know specifically matter for my child. I can't realistically fault them on this, because their behaviour management works fine for the majority of the children, but it is frustrating to have people assume that my behaviour management is at fault and I am looking for some kind of excuse or easy way out.
I would love a kind person in his classroom. I would have happily opted for that over a diagnosis. I thought maybe the diagnosis would be the way to get understanding and communication into his classroom - the system as it is does not seem to be working well for anybody. (I am not in the UK - so I don't need UK specific advice but I would guess that the distribution of parental behaviour is likely similar.)