Well, I managed to tolerate my son's neglect right up to the beginning of Y5 but there's only so much harm you can witness before you have to step in.
He had obvious problems even before entering Reception (ignored by HV and her 'wait and see' policy - I've had several of those), school were by their own admission 'very concerned', Head at one point described him as the most violent child with AS she'd ever known and acknowledged his danger to others but still refused to support an application for statutory assessment.
I moved him to another school with an ASD unit at the beginning of Y4, he settled initially (new kid in town, bit of a novelty, then the bullying started) but the crunch came when he started Y5 and had a vile, hideous ignorant witch for a class teacher who refused to tolerate him. It became apparent then that few of the 'highly trained' staff had a clue how to work with autistic children. In fact, in his whole five years in school he got no support at all, despite reports from Autism Outreach that clearly outlined his various needs and difficulties. He floundered for years and both schools refused to engage with me - once he was through the doors in the morning he was nothing to do with me .
My own experience was that the system is against children and parents rather than for them. There is more effort put into proving that a child doesn't need support than in providing it, finance and statistics drive provision in a worrying number of cases. Even those who do get help have a heck of a battle to get it - nothing comes easily and your child could be suffering while you're fighting.
My son was being harmed and neglected and I couldn't bear it any longer - God alone knows what it was doing to him. I deregistered when he was almost 10 (beginning Y5) and we've never looked back. I've been able to spend more time and effort on developing his social, emotional and communication skills in ways that wouldn't even have occurred to SENCOs, teachers, heads or the LEA. He is being supported appropriately and is now thriving - still autistic, still with problems but we have a consistent method of tackling them that is showing good progress.
One thing I should say is that a lot of people comment that it 'must be hard work' to home educate and yes, it is. But it's nothing compared with the stress of dealing with an education system that didn't want to help him or even protect others from his rages. We're out of the system more or less completely now and it's the most stress-free I've ever been since he was born .