This is mostly for those who think "well, if the child with SNs can't cope, he needs to go into a specialist school." Let me just give you an example.. and this is not even an extreme example....
DS1 was in mainstream school for Reception and Year 1. Within 3 days of him starting Reception, I was called into the Senco's office and told there was a concern that he had special needs of some sort. I literally sat and cried with relief - we had been bringing up our concerns with the various GPs at our surgery for the last 2 years, only to be fobbed off repeatedly and told "oh children develop at different rates, blah blah."
Senco recommended we go to GP and get referral to paediatrician for assessment. In the meantime, school puts 1:1 with DS1 fulltime, out of their own budget (while applying for additional funding as well) because they felt he needed it. We were lucky. Not many schools will do that with no statement.
GP refused to refer to paed, as she stated that ASD and ADHD (our concerns) were "mental health issues" and referred us to CAMHS, who promptly fired back a letter to GP stating she was basically off her rocker as DS1 obviously needed to see a paed. We got a referral from the school nurse and DS1 finally saw a paed, who then referred him to other appropriate specialists (SALT, OT, PT, etc).
Anyway, school assisted us in applying for a statement. Again, we were lucky - our school was brilliantly proactive and bent over backwards to support DS1.
By the beginning of year 1, it was becoming clear that DS1 was not coping in MS school. He had sensory problems as well, and because he could not cope with the noise and movements, he had to leave the class up to 20+ times per day to calm down. Thankfully, the children were very tolerant of his needs and treated him very well. (Some of the parents, however, were a different story.
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We spent that entire year fighting with the LA to get him into a local specialist school that stated very clearly that they could meet his needs. He is very clever, on G&T lists, and the LA said no way could he go to specialist school as he was "too able." However, they claimed that he could cope with the MS classrooms, and when they sent a woman to the school to discuss it with myself and the Senco, the Senco recommended she view him in the classroom so she could see how much he struggled - the woman refused. She said "it's not part of my job to see him in class." 
We had to appeal their refusal, prepare for tribunal. We had paperwork from literally EVERYONE that worked with DS1 (paed, OT, SALT, teacher, TAs, headteacher, Senco, etc) stating that he would do better at the specialist school and still the LA refused. They finally agreed on the very last day of school of Year 1, which of course meant DS1 wasn't able to go through a transition time to the new school (which was recommended by the paed, the specialist school, and his MS school).
He is 7yo and happy at the specialist school now. There is no way he could have dealt with junior school in MS.
Does this even clarify it a little?? We fought and fought, had the backing of literally everyone working closely with him, and STILL the LA refused and wanted him in MS even though everyone was telling them he couldn't cope with it. Do you have any idea what it's like to see your 6yo falling apart in front of you and not being able to do anything about it? We HAD to send him to school, knowing he couldn't cope, and being utterly thankful that at least his MS school was doing their best to minimise the damage to him.
And by the way... he is DISABLED, he is not a "special needs child." Just to clarify.