The main thrust of this opinion is that while a lot of thought is given to what is best for the special needs children themselves, very little thought seems to go into what effect placing them in mainstream school will have on the rest of the children in the class they are in. There are many cases where the child themselves is not disruptive to the class but requires a lot of help. Here it is a comparatively simple problem of evaluating how much resources the child will take up vs how much funding for that there is. If the child needs full time help but the LEA will only fund mornings for example then the school must divert one of it's ordinary classroom assistants the rest of the time. Can it afford to do so? How much is it disadvantaging other children in the school to devote this time specifically to one child?
These are difficult questions but in these cases I tend to lean towards mainstream school if possible. The cases where I feel special schools often should be used but aren't are those where the special need comes with behavioural difficulties. Here, it's not only the problem of diverting adult time to focus on one individual, although that often features as well, but the fact that that one individual has a negative impact on the whole learning environment. Even with near constant supervision, some children can be extremely disruptive, if not violent. It also can make the social aspects of the class difficult. Teachers ask the other children to be understanding, to put up with it, often referring to behaviour we would never put up with in an equal. Perhaps the disruptive child is benefiting from being socialised but the rest of his class aren't. Their education is suffering to give one child a better chance. It's not even a question of whether that child can help it or not - it's just not fair to those children who have to put up with it. But because the child is able to learn in a mainstream school, they won't get a place in a special school because that's expensive and an LEA will only fund it if there's no alternative. It's an open question what is better for the child in question (and will, of course, vary) but given that the staff-pupil ratio is much lower in a special school, I suspect that they would perhaps do better with their studies, if not with their socialisation.
Of course, if I had my way, it would be a lot more expensive. Special schools cost. But I think we would see benefits not only for the children who are sent there, but for the children who stay behind too.