Part of what kids learn at school is how to obey society's rules even if both kids and parents disagree with them
As a teacher, this is something I wholly disagree with. Kids aren't robots.
Parents need to accept that their kid of 6 or so must knuckle down like everyone else's and do as they're told
No, disagree with this, sorry. Education is evolving. School is about developing a whole person, not "knuckling down".
Backing the kid up against the school is disastrous in the long run; when the kid is a teenager they'll have no respect for authority and the parents' lives will become REALLY hard
Again, I disagree. Children should learn why rules exist, and which ones should be challenged. They need to be critical and ask questions. They need to question the education system and what it stands for. They need to learn how to be a good, kind and decent member of society, and that some rules are ridiculous and shouldn't be blindly followed.
Thankfully, educational research is also going down this route too. You'll start to see systems evolving. In many of the "top" countries, children don't wear uniforms and ridiculous rules don't exist.
The issues in this country, lack of respect, deprivation etc are not caused by blue hair or nail varnish.