Funding an extended school day is likely to yield “consistent and strong returns”, new analysis suggests.

But the outcomes of keeping children in school for longer depend on “how the time is used” - with the “most effective” methods drawing on “existing and well-trained staff” and “integrated to existing classes and activities”, according to the Education Policy Institute (EPI).

In June, Sir Kevan Collins quit his post as education recovery tsar after his plans for a £15 billion catch-up package, including extending the school day, were not fully backed by the government.

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Now EPI research fellow Luke Sibieta has concluded that the evidence on extra school time is “quite strong, both from the successful examples and from the less successful ones”.

“It is no secret that Sir Kevan Collins’ recommendations for close to £15 billion of education recovery spending were rejected by HM Treasury on the basis that the evidence for extending school time was weak and inconsistent,” he said.

“I argue that the empirical evidence is by no means weak. In fact, there is strong evidence that extending school time can improve test scores, if extra time is an extension of normal teaching practices and the curriculum, draws on school staff and is linked to schools’ existing activities in as many ways as possible.”

The “ultimate success” of any extension will be “heavily shaped by attendance levels”, said Mr Sibieta, who added that it may be beneficial to frame extra time after normal school hours as focusing, at least partially, on sport and the arts, which might be “more appealing for pupils”.

“There is also a clear risk that without extra time, such extracurricular activities might get heavily squeezed out of normal school time, if schools are more focused on catching up on literacy and numeracy levels,” he said.

Mr Sibieta concluded that extra school time is most effective when it “draws on existing and well-trained staff”, is “integrated to existing classes and activities”, and is “based on sound empirical evidence”. It is also more effective for maths.

“Providing this approach is followed, extra funding to enable extended school time is likely to yield consistent and strong returns,” he said.