I quite agree that we need more SEN schools.
When I started out teaching - not in the school that I mentioned above - my school had a separate Special Education Unit with its own HT. This took the children through all stages of primary and secondary.
The school moved into a new build. The SEU became the SE Department. Sometimes, it was possible for children from SED to take classes within mainstream, whilst supported by their assistant from the SED.
Some years later, that SED was only accepting non-verbal children, with a push to place more and more children into mainstream, but without the necessary resources - a theme right across our LA, with specialist facilities being shut down.
By the time I retired, I'd have a full class of 30 with one Classroom Assistant for the entire class - but only for about half the timetable for that class, if I was lucky.
The C.A. would become frustrated because it was literally impossible for them to work with children who needed their input. I'd be running around trying to support children working at umpteen different levels, taking different courses and dealing with educational and behavioural problems across the board.
In the meantime, the C.A. would finds themselves unable to work with children who would have benefited from the extra support because no one could get any work done unless the child with the most severe behavioural problems was getting one-to-one.
On top of the usual difficulties, we had some pupils with English as a Second Language. Support from the LA consisted of one morning visit a week from an ESOL teacher - for the entire school.
Until the powers that be are prepared to pay for more staff, we're all stuck between a rock and a hard place.