Maisy, Poor behaviour is regarded as an additional need though (especially at primary age) and as such should be supported by schools, regardless of whether at that point there is diagnosed SEND or not, children with disabilities are not always diagnosed by the age of 4 when the majority start school, many children are not diagnosed with SEND until they are late primary or early secondary age or even beyond, and that is often because schools do not have a good knowledge or understanding of SEND, this is of course in my experience.
Two children completely different profiles one with a diagnosis of Asperger's Syndrome at 6yrs old, school said they "can't have LD with that diagnosis", but he had a statement with 25hrs 1-1 based on the findings of a SaLT report and poor behaviour. Fast forward a few years and he ended up in a special school where academic difficulties were recognised and reasonable adjustments made leading to a massive reduction of the challenging behaviour, fast forward a few more years and at 16 the true extent of 2 specific learning disabilities came to light. For far too long my concerns were brushed aside with a "we are the professionals", "we have expertise in this field", "we know what we are talking about" as for me I'm just a mum who sat with her child night after night for 6 years while he had night terrors, he and I survived every night on short bursts of sleep. He was terrified of school because his needs were not being met. He was excluded because he couldn't not wouldn't behave. He was made to suffer consequences because he couldn't not wouldn't conform to expectations placed on him by a school who wouldn't meet his needs.
The other child coasted through infants and juniors until yr 6, then began to struggle slightly in 1 subject and that continued to worsen throughout secondary but was not flagged as a problem, left school and went to college but struggled massively for the first yr, college got their EP involved and a specific learning disability was diagnosed and also the fact that she was in the top 2% on other parts of the Assessment, she was allowed to re sit the first yr with Support and other reasonable adjustments. There has never been any challenging behaviour but she went through school and college masking her difficulties and is now considering persuing an ASD diagnosis because she totally fell apart at work. I would not be surprised if she got the same diagnosis as her sibling.
Schools must not disclose medical information to all and sundry so often parents of other children assume there are no disabilities or other difficulties but see certain children being treated differently and threads like this appear.