Last Friday, Ofsted published reports on six private Islamic schools in East London that were among the most critical and well-argued I’ve ever read. The schools were described as “inadequate”, and the reports listed failings ranging from low achievement to the promotion of social attitudes incompatible with modern society.

In some schools, huge chunks of a curriculum most people would see as essential were simply left out – no art, no music, and a view of history dictated by a single religious viewpoint. The Guardian carried an article condemning the reports by the former pupil of one school (Jamiatal Ummah) who had proceeded to the physics department of Kings College. He would have slim chances of getting there now – the school’s data dashboard shows no pupils with ABB grades at A level, and an average A level grade of D+.

Published on the same date were two reports on state schools, Globe Ark Academy near the Elephant and Castle in South London, and Sir John Cass School, in the East End. The Globe report, like those on the independent schools, is a first-class piece of work by the inspectors.

The school’s work is given a fair and balanced analysis, with due weight to strengths and aspects of improvement, and its overall rating is good, with outstanding features in management, behaviour and, perhaps most important of all, achievement in the primary school, showing that the approach pioneered at Mossbourne can be both exported and extended.

The school has been open for two years only, and this is what excellent work in progress looks like. It is also a huge improvement from the situation in that part of London when I worked there in the seventies.

Sir John Cass, by contrast, is entitled to feel it has had a raw deal. The report does not conclude, as it rightly does with the six private schools, that “the school is inadequate”, as the evidence of pupils’ work in the main school and the sixth form clearly shows that it is not.

Teaching and achievement across the whole range of the school’s pupils are good, GCSE results are above national averages and outstanding in English and maths, with outstanding provision in vocational courses in the sixth form – a key issue for improvement nationally. Pupils behave well, work hard, and have above-average attendance. This from the East End, and the neighbouring school to the one I happened also to work at in the seventies. Once again, the improvement is remarkable.

Sir John Cass had two shortcomings. It did not monitor the Facebook site of a sixth form society that was promoting extremism, and did not ensure that children in the main school mixed freely in the playground. Both of these are serious and required immediate attention from the governors and headteacher. Inspectors could have made this crystal-clear, listed it as a point for urgent action, and promised that they would be back very shortly to check that the necessary steps had been taken.

Instead, they – or someone in Ofsted’s central office, as the report was published six weeks after the inspection – swung the sledgehammer, downgrading the school from Outstanding (2011) to Inadequate, and, in effect, wiping out its good work. This is, to use Ofsted’s own terminology, a seriously misleading report. The judgement does not properly reflect the inspection evidence, and puts a conscientious school in the same category as those which were clearly failing to educate children properly. It is unfair.

The problem is rooted in the changes Labour made to Ofsted in 2005, which have still not been put right. In pursuit of Blair’s notion of joined-up government, Ofsted was made to inspect social services as well as education, and the inspection of schools was slashed to the point at which inspectors did not have time to do their work properly. As one retired HMI has put it to me “they couldn’t find the evidence to substantiate their judgements.”

The only way to make this system appear to work was to use data instead of first-hand evidence, and data under the Labour régime were full of perverse incentives either to cheat or to exercise discretion in a way that made your school look better than it should.

So, results in stand-alone infant schools tended to be better than those in primary schools, which had an incentive to keep the infant results low to make it look as if the juniors were making good progress. If they didn’t, some data-minded inspector would come and fail the junior school without looking at its work. Or praise it on the same basis. Sir John Cass School was not visited at all when Ofsted rated it outstanding in 2011, and had only one day’s inspection, by one inspector, in 2008. Its last full inspection, under a properly comprehensive framework, dates from 2005.

In the same week as the reports that made the headlines, Ofsted published just under 250 reports on the work of individual childminders and dozens of reports on other childcare, including fostering agencies and children’s homes. Such inspections are necessary, but it makes no sense whatsoever to visit every childminder in the country while sending a large secondary school like Sir John Cass a nice letter without setting foot in the place.

To use plain language, you can’t inspect a bus ticket without looking at it, and we have, since Labour’s sectarian decision to force Sir Mike Tomlinson to resign the minute he was sixty, had a succession of chief inspectors whose experience of the work of inspecting schools has been close to zero.

Leadership roles in education are not interchangeable, and Sir Mike remains the only professional inspector to have held the top job. There is, unfortunately, only one of him to go round. Now in his seventies, he has been asked to try to improve matters in Birmingham.

The issue is made more urgent by calls in some quarters to reduce inspection still further. Inspectors, according to a comment in The Guardian by a former staffer for Gordon Brown, should not be “food critics” – an excuse for reducing direct observation still further and leaving everything in the hands of school managers, who may have a conflict of interest and who have not been trained to observe fairly.

Senior leadership in Ofsted has abandoned the HMI tradition of expertise in subjects, so that specialist reports on subjects have been discontinued, and retiring subject leaders are not being replaced. This is Labour’s agenda, and deeply damaging to standards. It must be reversed.

The extremist threat to British education has left Ofsted, and those who have made it as it is, so seriously embarrassed as to threaten the credibility of the entire system of school inspection.  Ofsted has responded to this embarrassment by using inspection to make the point that it still has teeth. In doing so, it has put to one side the principles of basing all judgements on evidence, of having sufficient evidence to substantiate each judgement, and of treating teachers well and fairly. These principles need to be restored, and quickly.