I don't know what schools and teachers you've had contact with, Shed, but stating that teachers don't even try, or that schools won't back parents is a horrifying generalisation and something that I have never, ever experienced in many years of teaching and working in educational inclusion.
We have experienced this not in one school, but actually in two. One mainstream, one specialised - two different children. It happens. It shouldn't, but it does.
When I found out that my (then) 5yo's school was shutting him in an isolation room alone when he was having autistic meltdowns (due to lack of support as they refused to provide the needed support), I withdrew him the same day. I will note that the asst headteacher sat right in front of me that day and told me they put him in there as punishment for his behaviour, even though they knew it was related to SNs and even though they knew he was not receiving support. God knows we tried to get the support for him, meeting after meeting, bringing in Parent Partnership, contacting the LA, and finally going around the school to get assessments without their cooperation (they refused to fill out the paperwork and didn't assist with any referrals).
Three years later, he is still terrified not only of schools but of the door being closed in any room that he is in.
This was a school where he was bullied relentlessly, but the school stated it wasn't bullying because he fought back. Basically, the teachers were doing nothing to stop it,even after it was reported, so his anxiety reached a point where as soon as the bully started on him,he fell apart and lashed out. Then he got in trouble and put in isolation - a 5yo boy, screaming and crying, alone in a little room, unable to get out. We were told that if he didn't have SNs (which they acknowledged he had, but refused to support), they would have excluded him. Instead, they terrorised him by locking him in a room.
If he went back to school now, he'd likely lash out at anyone that upset him. His anxiety would be that sky-high. We home ed - it's better for him academically as well.
But... the other parent said my child was the bully because he hit her child.
The other situation is long and complicated, so I won't go into it here.
There are good schools out there, with teachers that go that extra mile. But there are also very poor schools - and it starts at the top. The school that this occurred at had a change in a number of admin positions, including headteacher and senco, about a year before this situation. This changed the way the school dealt with children with SNs dramatically. Prior to that, it was excellent. The difference was shocking. I wouldn't have believed it if I hadn't seen it myself.