Doubts have been cast on the quality of some of the country’s most sought-after schools after it emerged that hundreds graded outstanding by Ofsted have not been inspected for more than a decade and their assessments may be out of date.

A report by the National Audit Office (NAO) revealed that more than 1,600 schools teaching tens of thousands of pupils had not been inspected for six years or more, and of those, almost 300 had not seen an Ofsted inspector for at least 10 years.

The schools watchdog is legally required to re-inspect schools every five years, but those judged outstanding – many of which are grammar schools – are exempt from reinspection. Concerns have thus been raised about the continuing accuracy of the judgments on which many parents base their choice of school.



It is the first time the NAO has investigated the schools inspectorate, and its report is highly critical in places, concluding that Ofsted cannot demonstrate that its inspection of schools represents value for money, and does not know whether its inspections are having the intended impact of raising education standards.



According to the report, over the last four years, Ofsted failed to meet its statutory target to reinspect schools within five years in 43 cases. In each case Ofsted has offered an explanation. It also failed to meet its own target of reinspecting inadequate schools in 78 cases over the same period.

The report, published on Thursday, shows time spent on inspections diminishing and periods between inspections extending. According to Ofsted’s own targets, good schools should be reinspected every three years, but in 2016/17 this period increased to four years. The maximum time for reinspection of schools graded as “requires improvement” – and therefore in need of closer attention – was extended from two years to 30 months, but 55 schools still waited longer.



Ofsted also missed its target of inspecting new schools within two years of opening in 95 cases between 2012/14 and 2014/5, and extended the target to three years.



The NAO acknowledged that as across much of the public sector, Ofsted has been subject to significant funding cuts in recent years, which have seen spending on school inspections down 52% in real terms over the last two decades despite increased responsibilities.



As a result, the watchdog has switched from conducting in-depth, comprehensive inspections to a “snapshot” of standards, using a light-touch, risk-focused model of inspection. A good primary school, for example, will be inspected by a single inspector over a day. Resources are concentrated on underperforming schools, leaving high-achieving schools unchecked by routine inspection, as long as data shows that they are maintaining performance.

“The length of time without inspection means that some pupils go through primary and/or secondary without an independent assessment of their school’s effectiveness,” the report states. “The older an inspection judgment, the greater the risk that it is no longer accurate. This reduces the level of assurance available about the school concerned.”

Amanda Spielman, head of Ofsted. Photograph: Graeme Robertson/for the Guardian

Responding to the report, Ofsted’s chief inspector, Amanda Spielman, said the organisation was operating in a difficult financial environment. “This means that we have had to make tough decisions about how we prioritise resources. I am confident that Ofsted gets the balance right. For example, by focusing more resource on inspection of schools that are less than good. An increase in either the number of inspections or time spent on inspection will quite simply require greater funding.

“Ofsted is only one lever in the school system, which is why it has proven difficult for the NAO to judge our impact and value for money. As we have made clear to the NAO, judging ourselves against school outcomes would inevitably create perverse incentives.”

Ofsted has found it difficult to meet its targets in part because it lacks sufficient inspectors. Many have been poached by multi-academy trusts offering salaries in excess of £100,00 compared with the £70,000 an HMI will receive. The turnover of inspectors has improved, but is still high at 19% a year.



Launching the report, the NAO head Amyas Morse acknowledged Ofsted’s strengths: “Ofsted’s role as an independent inspector is valued by parents, headteachers think its judgements are fair, and it is making headway against recent performance shortfalls.

“However, it needs better information to be able to demonstrate that its inspection of schools represents value for money. The Department [for Education] needs to be mindful that cheaper inspection is not necessarily better inspection. To demonstrate its commitment, the department needs a clear vision for school inspection and to resource it accordingly.”

An NAO survey found that three in 10 headteachers (28%) believed inspection had not led to improvements in their school, compared with 44% who thought it had. Seven out of 10 respondents said inspectors had provided useful feedback.

Teachers’ leaders who have long criticised Ofsted welcomed the report as confirmation of their view that Ofsted was not fit for purpose and called on ministers to conduct a root and branch review of the system of holding schools to account.



Mary Bousted, the joint general secretary of the National Education Union, said: “The revelation that Ofsted does not know whether its school inspections are having the intended impact, to raise the standards of education and improve the quality of children and young peoples’ lives, is devastating.”



Geoff Barton, general secretary of the Association of School and College Leaders, said: “We agree with the head of the NAO that cheaper inspection is not necessarily better inspection, and we support the recommendation for a review into whether Ofsted has the resources to do its job.”