Hi Jean (sorry, couldn't resist that rather old joke!!

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I cannot discuss your questions as an 'expert'; just as a mainstream teacher who has a brother and sister who were educated in Special Schools (as Down's) until they were 18, at which they started an FE college with excellent facilities. There were NO facilities from their 5-11 age that were anything like as good as the special school, and by the time they ought to have attended an 11+ school, my mother was the one institutionalised on their behalf! So they did not enter - to my regret - mainstream ed until they were 18. Too late for my shy sister, but great for my more ebullient brother!
Money dictates what we do here - and where you are too I imagine. Your country's circumstances are different, so perhaps it is cheaper to integrate there? Please explain. Your questions come from some belief you have and I am interested what prompted them.
Anyway, imo, here it is cheaper to staff one or two special schools per Ed Authority and bus the children in rather than pay individual specialist staff here and there to go to our many mainstream schools. Hence it is more common to find more secondary schools than primary offering integration to some special needs children, as by definition primary schools are smaller and there are more of them, thus they would require more people to deal with individuals.
I am sure you know all this.
I also live with the consequences of this economic decision as my sister will now never be integrated into mainstream society and will always be highly dependent; as opposed to my (potentially) moderately independent brother who is currently doing his best to learn belated social skills. My brother is managing,in a limited way, to adapt, my poor sister needed to be socialised from an earlier age than eighteen to have helped her.
But that does not mean that I believe that ALL special needs students should be integrated. Far from it. I have direct experience of my siblings' peers at their special school and it is clear that some people will be forever unable to interact socially with others. In that case it is cruelty to subject them to enforced socialising (as they will experience in mainstream). People with such severe problems will either not be able to recognise social interaction, or will be some sort of personal hell because of it. The only reason I can see anyone might have for forcing integration on these young people is a- to save money, or b- to provide tolerance skills for so-called normal students. Neither imo justifies forcing severely disabled young people into situations which we would not enjoy were we in their place.