Ofsted is straying from its brief and setting educational policy through its new inspections, according to a report backed by former education secretary Nicky Morgan.

The inspectorate has been told not to undermine academy freedoms or substitute its judgement for those of successful headteachers, in a report by the Policy Exchange think tank.

It also claims that Ofsted has a “de facto preference” for schools to run key stage 3 over three years, and reveals that schools which have done so are more than twice as likely to be rated as good or outstanding under the new inspection framework than those who shorten it and run GCSEs over three years.

It follows a major row between high profile MATs and Ofsted over the way in which the watchdog inspects the curriculum at key stage three and GCSE.

Exclusive: Heads fear Ofsted will penalise three-year GCSEs

Curriculum row: New research to assess the impact of three year GCSEs

Opinion: MAT leaders say Ofsted inspections will harm the pupils it wants to help

The report warns that Ofsted is straying beyond its remit, on both the curriculum and faith schools.

The report, which has been backed by Nicky Morgan, also says Ofsted should not be assessing the curriculum of academies against the national curriculum.

In a foreword to the report, Ms Morgan says: “Part of the success of academies, if they choose to make use of them, are the freedoms they are given. We must be careful not to undermine those academy freedoms by the back door and it is important that all of us, including Ofsted respect that.”

The Harris Federation and Outwood Grange have suggested that the new inspections have penalised schools who chose to shorten key stage 3 and run longer GCSEs.

Ofsted has said it does not have a preferred key stage 3 length.

However, the Policy Exchange report says that the wording of Ofsted’s new inspection framework has created a preference for schools to run key stage 3 over three years.

It says than an analysis of the inspection outcomes shows that those which run a longer key stage three have done better. 

A table in the report shows 16 per cent of schools running a key stage 3 over two years were rated as inadequate compared with 7 per cent of schools that ran key stage 3 over three years.

And 24 per cent of schools which ran a key stage 3 over two years were judged to be good or better compared with 56 per cent of schools which ran a three year key stage 3.

It calls for the watchdog to delete a line from its framework which says: “If a school has shortened key stage 3, inspectors will look to see that the school has made provision to ensure that pupils still have the opportunity to study a broad range of subjects, commensurate with the national curriculum, in years 7 to 9.”

One of the report’s authors, Iain Mansfield, said: “Ofsted is of critical importance in our school system. It is absolutely right that the new inspection framework assesses the curriculum, but this must not be at the expense of the hard-won increase in exam results, particularly for disadvantaged children.”

This echoes comments made by the leaders of the Harris Federation and Outwood Grange who suggested that Ofsted’s new curriculum focused inspections were part of a “middle-class framework”. 

The report also says that Ofsted should not benchmark the curriculum of academies against the national curriculum as academies do not have to follow it.

The Policy Exchange does back Ofsted’s crackdown on ‘gaming’ schools.

However, it adds: “While inspectors are right to critically examine the curriculum, in the absence of ‘gaming’ they should be careful not to substitute their judgement for that of headteachers who are producing good results.

“Where freedom and flexibility has been granted by Department for Education policy, whether to academies, faith schools or elsewhere, Ofsted should be careful to ensure it is not reducing that freedom, either as a matter of policy or inspection practice.”

An Ofsted spokesperson said: “We want schools to achieve good results by doing the right things. This report underlines Ofsted’s critical role as ‘the guardian of standards, [and] the champion of pupils’.

“We welcome its support for our new inspection framework and its backing for our strong stance against schools that game the system. We’ll continue to celebrate excellence in all types of school and to recognise academies and faith schools for exercising their freedoms in the best interests of their pupils. We’ll also continue to listen carefully to feedback on our work.”