In 1968 there were no middle schools in the United Kingdom; by 1978 there were 1,690. Five hundred opened in 1973 alone. Today, there are just over 400 left. Just as they were squeezed into existence by the tectonic plates of politics, demographics and simple "bums on seats" - local authorities had to accommodate a new comprehensive system in existing buildings because there was no extra money - so now the plates are on the move again. This time, it's surplus secondary places that are causing middle schools to fall down the cracks. Educational principles - although they are often conjured up for post-hoc justification - really don't come into it.
Middle schools are divided into 8 to 12s - effectively primary schools with some extra resources - and 9 to 13s. The former are declining fast; the latter - there are about 300, and Northumberland, with 45, has the most - are different and distinct, and there's a strong hope that they'll survive, even though they find themselves permanently engaged in consultations about their continued existence.
Studies have overwhelmingly concluded that middle schools do an effective job at both key stage 2 and 3. The notorious Year 7 dip tends not to happen. This is seen by heads as one of the strengths of the middle school.
Nigel Wyatt, the former head who is executive officer of the National Middle Schools Forum, says, "We know that transfer at 11 can be disruptive and there's evidence that transfer at 13 is less so."
He points to a 1999 York University study that showed pupils from middle schools making better than average progress between KS2 and KS3. Now that value-added data is available, he's analysing performance and is in no doubt that the figures will show children who transfer at 13 make better progress than those who arrive in secondaries at 11.
John Kine, head of St Edwards, one of 14 nine-to-13 middle schools in Staffordshire, is very clear on the advantages of having this age group together, seeing it as a coherent developmental stage. "There's a golden age of learning between 9 and 13," he says. "It transcends key stages 2 and 3. The children haven't reached the stage where 'can't do' is part of the vocabulary."
Building on this, he says, "The governors and the staff have always valued very a broad and balanced curriculum. That's at the heart of what we do."
He believes that secondary schools, with the best will in the world, have priorities that lie further up the age range.
At the same time, say middle- school heads, it's an advantage for younger children to have the benefit of qualified specialist teachers. At St Edward's, specialist teaching starts in Year 5, the pupils' first year, and there's a system of setting in core subjects. This means, says Mr Kine, that by the time his children face GCSE in the high school, they've had seven, rather than five, years of specialist subject teaching.
"In terms of curriculum," he says, "we're secondary'; in social terms, we're a window of opportunity for child development."
Are the remaining middle schools going to disappear? Apart from strong parental support, the heads can see two chinks of light. One is the key stage 3 strategy's work on the "condensed KS3" (see below) which, for some middles at least, could give them complete ownership of both KS2 and KS3.
(League table judgments on middle schools are made on the basis of KS2 tests, even though their pupils are with them for two further years.) Another comes in the attention being paid nationally to the 14 to 19 curriculum, which could encourage secondaries to see the wisdom of continuing to run with just that age group.
Geology tells us, though, that tectonic plates are fearsomely hard to stop when they start moving.
York study on the National Middle Schools Forum website www.nmsforum.co.uk