The country’s largest academy chain is “failing too many pupils”, Ofsted has concluded.

In a letter published this morning, the watchdog said almost half of the Academies Enterprise Trust’s (AET) secondary pupils attended schools rated “inadequate” or “requires improvement”, and that the performance of the trust’s secondary schools was “mediocre”.

Four in 10 of its primary pupils attended schools rated less than good, Ofsted said. The regulator added that children from poor backgrounds did “particularly badly” in the trust – which runs 67 schools - and that attendance levels were “unacceptably low”.

But Ian Comfort, AET’s chief executive, said the trust’s schools were improving and that Ofsted had only visited a small proportion.

He told TES that he wanted a better process for inspections of academy chains and that he would not resign.

In the past two years the proportion of AET schools rated good or outstanding had jumped from 32 per cent to 64 per cent, and the proportion judged inadequate had fallen from 29 per cent to six per cent, Mr Comfort told TES.

“Could we move some of them out [of the inadequate category] quicker?" he added. “Probably not. You’ve got to have the right data. All of those schools are on a journey that we’re tracking.”

Mr Comfort suggested that parents were part of the reason for attendance that was “still not good enough”.

“There are particular schools in particular areas of the country where attendance becomes a challenge and where we have parents whose aspirations for their children aren’t what they should be,” he said. “We need parental support and we need to work with communities as well.”

This is the second time in less than two years that Ofsted has criticised the academy chain. In 2014 the watchdog said too many pupils at AET schools were not receiving a good enough education.

Today’s report notes that nearly three-quarters of AET’s current academies or their predecessor schools were judged to be less than good when they joined the chain.

Ofsted's findings are based on inspections of seven academies and phone calls between Ofsted inspectors and the principals of 18 other AET academies in November 2015.

The watchdog’s letter to the trust said that since Ofsted’s 2014 report, “in nearly a quarter of the monitoring inspections of secondary academies, inspectors judged that the academy was not improving fast enough".

Standards at key stage 4 were “not high enough, or rising quickly enough”, it added.

“A few secondary academies can point to marked improvements in pupils’ rates of progress and improved attainment,” the letter says.

“In others, however, attainment has dropped significantly. This calls into question the effectiveness and impact of the trust’s improvement strategies.”

It said the trust must improve the attainment of disadvantaged pupils “as a matter of urgency”.

But AET said the gap between disadvantaged pupils and their peers in the trust's primaries was narrowing, and that at key stage 1 it was smaller than the national average.

Mr Comfort told TES it was “quite possible” that the Ofsted letter could prompt the government to discuss using new powers in the Education and Adoption Bill to remove some academies from the chain.

But he said there were “no discussions at the present time about removing academies from our group”.

Asked whether he would resign in the wake of Ofsted's criticism, Mr Comfort said he had the support of AET’s trustees and headteachers and had “no intention of resigning at the present time”.

“I’d question the tone of the [Ofsted] letter because if you drop below the headlines you’ll see a number of good things being said,” he said. “We had a discussion [with Ofsted] about those not judged as good, and the journey those schools are taking.”

A Department for Education spokesman said: “We are determined that no child should have to spend one day more than necessary in an underperforming school. While Ofsted’s findings show that AET has made some progress, particularly in its primary academies, the trust is letting down too many of its pupils.

“The academies system allows us to spot and intervene in underperformance far more quickly than in council-run schools. AET must now show us how they will address the issues identified by Ofsted and we are working with them to raise standards. If we are not satisfied we will take further action.”

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