At the same time, never backwards in cruelty, kids themselves adopted 'special' as a term of intellectual derision. To be 'special' was shaming in a way that being 'slow' never was; there's no shame at being slow in Suffolk.
Then along came Labour with a performance target for 'Statementing', a made-up word to describe the production by the LEA of a Statement of Special Educational Needs for ESN and, er, 'slow' kids. Overnight, it became a mini-industry as bureaucrats raced to meet their Statementing targets.
Today 20% of the nation's school roll are classified as 'special', more than 700,000 kids. It's all, of course, an incredible waste of money. If a child is slow, no amount of additional attention will make it intellectually quick. We need slow people; we depend on them to do the tedious, repetitive and largely manual jobs as vital to the functioning of our society as brain surgeons. Accept it.