On the face of it, Ofsted is an organisation that is flying high.

Not a week goes by without a bold new announcement from chief inspector Amanda Spielman. Whether it be pledging to put multi-academy trusts under more scrutiny (see bit.ly/OfstedAcad), tackling the “off-rolling” of pupils or focusing on disruptive behaviour in schools, the inspectorate has appeared firmly on the front foot across a range of issues.

Meanwhile, the watchdog’s central role in the school system looks more secure than ever. When education secretary Damian Hinds announced that it was only Ofsted - and not his own regional schools commissioners - that should be inspecting individual schools, it cemented the organisation’s place at the top of the accountability tree (see bit.ly/OfstedTop).

But look beyond this Whitehall victory and questions are mounting about the whole purpose and value of school inspection. Concerns have been raised about the reliability of Ofsted’s grades, the validity of its inspection process and why there are more than 1,500 schools that have been left uninspected for many years (bit.ly/NotInspected).

There is a charge of over-reliance on data and claims that Ofsted is systemically biased against schools in poor areas. Some argue that the whole approach is wrong and that the inspectorate is part of a high-stakes accountability system that has now outlived its usefulness.

And if inspections are inherently unreliable, and too short to produce a fully rounded picture of a school, what is the point of conducting them? If an Ofsted report now tells you little more about a school than the performance data that is already published, what is the point of Ofsted?

The inspectorate has come to be seen as a foundation stone in our schools system - relied upon by parents and politicians alike. But it is worth remembering that England went for decades without Ofsted, and that school inspection is done very differently elsewhere in the world.

Dr Melanie Ehren, of the UCL Institute of Education’s Centre for Educational Evaluation and Accountability, points out that in a high-performing jurisdiction such as Finland school inspection does not exist. And in some areas of Germany, she says, policymakers decided to abandon inspections because they did not see a benefit.

Closer to home, school leaders in Wales have been told they will be given a year’s break from inspection and then move to a system where schools are not given graded inspection judgements because of concerns about their negative consequences.

Dr Ehren researched seven European countries and found that England had the most “high-stakes” approach for school leaders when it came to school inspection and accountability.

It’s not a popularity contest

Ofsted’s job isn’t being popular, however, and the embattled organisation has survived criticism in the past.

During Sir Michael Wilshaw’s era, there was a question over the whole future of the inspectorate within the Department for Education, and there was also a debate about whether Ofsted could accurately assess schools.

Professor Rob Coe, of Durham University, suggested that its school inspections were neither research- nor evidence-based. In 2013, Coe asked what evidence there was that the watchdog’s inspections led to “valid” judgements or that inspections created a benefit to the system .

Characteristically, the chief inspector of the day dismissed this scrutiny as “a lot of tosh and nonsense”.

Today Ofsted strikes a more collegiate tone. It has invested time and effort “myth-busting” in an attempt to stop schools mistakenly changing what they do to meet the needs of inspections. And its national director of education, Sean Harford, regularly engages with the profession on social media.

Last year Ofsted even provided an answer to one of Coe’s key questions by publishing a short study, which concluded that 92 per cent of its short school inspections were reliable (bit.ly/OfstedTrials).

However, since then Ofsted’s own head of research, Daniel Muijs, has admitted that it is impossible to determine conclusively how reliable the watchdog’s school inspections are (bit.ly/OfstedProof).

And now the reliability questions hanging over Ofsted have been revived by a National Audit Office investigation report on the inspectorate.

Just over a month ago, the government’s spending watchdog warned that the level of assurance that Ofsted provided about a school’s effectiveness had reduced - because the inspectorate’s budget has been cut.

The majority of schools now get only a one-day inspection from Ofsted, which, as the NAO noted, “allow inspectors less time to discuss with schools how they might improve”.

Meanwhile, Spielman has admitted that Ofsted no longer has time to carry out “full scrutiny” of many of the schools it visits.

The consequence of a reduced Ofsted budget is both shorter and fewer inspections.

The NAO report highlighted how making “outstanding” schools exempt from routine reinspection meant 1,620 schools had not been visited by Ofsted for six years, and 296 had not been seen in over a decade.

Ofsted itself warns that this situation is unsustainable and has said it wants “outstanding” schools’ exemption from inspections to be removed (bit.ly/NotInspected). However, it will need more money to inspect them.

All of which leaves Ofsted at a crossroads, according to Nick Brook, deputy general secretary of the NAHT headteachers’ union.

He says a decision needs to be taken to either increase Ofsted’s funding or to focus the resources it has on supporting the relatively small number of failing schools where intervention is needed.

Coming to a decision

He questions whether the current system allows Ofsted inspectors to uncover anything new about a school. One-day inspections, Brook says, are not long enough to be useful but have become the norm.

“A colleague told me how his school had recently had a one-day inspection and the inspector was literally having to form judgements on the move as he walked through the school,” Brook adds.

Ofsted insists that it can carry on as it is and is content with the level of assurance it provides of school effectiveness. However Luke Tryl, Ofsted’s director of corporate strategy is also clear that any further cuts will have an impact.

When Ofsted first announced one-day inspections for “good” schools, it was suggested that this would make it less “high-stakes” and more light-touch for schools. However, Brook, who is leading an NAHT commission on school accountability, says that this policy may have added to schools’ workload.

“What we have seen from the commission is that this is creating a lot of work for schools that are on the borderline between ‘outstanding’ and ‘good’ or ‘good’ and ‘requires improvement’. If you are a school where the data doesn’t point 100 per cent to you being ‘good’ or ‘outstanding’, then the onus is on the school collecting and presenting evidence to make that case.”

His point also demonstrates the inescapable link between data and inspection outcomes.

When Spielman took up her post last year, she told Tes that Ofsted was valuable precisely because data could never tell you everything.

But inspectors’ ability to look beyond results in reaching their judgements is the subject of fierce debate - fuelled by the reduced time inspectors spend in schools.

The fact that more than four out of five grammar schools are rated as “outstanding” while only around one in five non-selective schools get the top grade appears to some to prove that it is pupil intake rather than the quality of a school that really drives Ofsted ratings.

However Harford says those who claim that data drives inspection reports “conveniently overlook” the evidence contradicting their view. He highlights Progress 8 as an example of this.

“In schools with a Progress 8 score range of 0.0 to 0.3 there are schools graded as ‘outstanding’, ‘good’ and ‘requires improvement’ - three of the four inspection outcomes.”

In recent weeks Ofsted has faced a new charge: that its reliance on data has biased it against schools in deprived areas, particularly those serving white working-class communities.

Data from Ofsted itself showed that out of schools with the fewest white pupils from deprived backgrounds, 58 per cent were judged “outstanding”. In contrast, out of the schools with the highest proportion of white pupils from deprived backgrounds, just 4 per cent were given the top grade.

Some headteachers suggest that this trend is being driven by schools’ Progress 8 scores, claiming that this performance measure penalises schools serving white working-class children (bit.ly/OfstedWhite)

Ofsted has strongly refuted claims that it is biased against schools in deprived communities. Spielman herself has praised those teachers who make it their mission to tackle disadvantage but warned that “the overall effectiveness of a school is not an effort grade”.

In short, Ofsted has to call it as it finds it. And it is a crucial part of Ofsted’s job to identify underperformance.

However, academy boss Frank Norris - himself a former senior Ofsted official - says that inspection data points to evidence that the current system is delivering an unintended bias.

Home advantage

Norris, director of the Co-op Academies Trust, believes Ofsted does not draw enough of its inspectors from schools in deprived communities - staff who can understand the context of the schools they are assessing.

He claims that the current system does not “allow inspectors to get under the skin of the area and find evidence that explains the massive positive impact schools in disadvantaged areas provide.”

The fierce debate about the future of Ofsted is driven not just by concerns around the validity of its inspections, it is also about the consequences.

After the publication of the NAO report, Mary Bousted, joint general secretary of the NEU teaching union, suggested that the inspectorate needed to be scrapped.

She warned that it was a key player in a school accountability system that was driving teachers and school leaders from the profession, exhausted by stress and excessive workload.

But Harford points out that Ofsted’s job is to provide an objective view on the quality of education in a school. He says that questions about any punitive consequences that follow an Ofsted judgement should be directed at the people taking those actions.

Although heads’ unions are more positive about the need for Ofsted, an early finding from the NAHT’s commission on accountability was that the current system does not benefit the majority of schools.

The NAO report suggested that these claims might have some justification.

It said that inspectors themselves had found that shorter inspections made their jobs “more about checking compliance and less about improvement and follow-up work”. So if Ofsted inspectors are questioning the value of their work, who benefits from the current system?

The obvious answer is politicians and parents. The DfE demonstrates this with its reliance on its go-to statistic on education: that there are now 1.9 million more children being educated in “good” schools than there were in 2010.

Ofsted judgements help to provide a soundbite about a government delivering on education improvement - albeit one that has been questioned recently by the Education Policy Institute.

Parents also look to Ofsted to inform them about the quality of education on offer for their children. And what politician would risk abolishing something that parents value? That has been the received wisdom, at least.

But a poll this year by Parentkind, formerly PTA UK, showed that while just over half of parents had looked at Ofsted reports, only a quarter said the report was important to them when choosing a school place for their child.

Parentkind’s acting chief executive, Michelle Doyle Wildman, believes that having an independent inspectorate is important to parents, and she describes Ofsted as having been a “force for good” in the education system.

However, she says there is a need for a debate about whether it is fit for purpose in terms of what the school system needs now.

It’s not all been bad news for Ofsted. The NAO report included some positives with the vast majority of heads it surveyed - 84 per cent - saying they were happy with their inspection. And more (44 per cent) said that inspection had improved their school, than those who said it didn’t (28 per cent).

But such findings are unlikely to allay deeper misgivings about Ofsted.

During the height of the tensions between Wilshaw’s Ofsted and the DfE, a leaked memo from former Michael Gove adviser Dominic Cummings suggested that they might need to “look at Ofsted with a blank piece of paper”.

Nobody is suggesting this now. But if the government were designing a system from scratch, it is difficult to imagine that anyone would propose what we have in place today - a schools inspectorate without the money to carry out its mission and major doubts within the teaching profession about whether the mission is even the right one.

In its conclusion, the NAO noted that the government has reduced Ofsted’s budget for more than a decade while asking it to do more.

The answer it puts forward is a simple one: “Government needs to be clearer about how it sees Ofsted’s present and future inspection role in the school system as a whole, and then resource it accordingly.”