Attending a grammar school has no positive impact on a teenager's self-esteem or their future aspirations, a study has suggested.
Grammar school students also do not gain any academic advantage over children who attend non-selective state schools by the age of 14, researchers have found.
The study, by the UCL Institute of Education, comes just days after the government announced controversial plans to hand over £50m to existing grammar schools to create more places.
Researchers analysed data from 883 children in England and 733 children in Northern Ireland who had similar academic achievements at primary school and came from similar backgrounds.
And they found that attending a grammar school had no positive impact upon teenagers' attitudes towards schools, self-esteem, future aspirations or their English vocabulary.
Those who led the research say the results indicate the funding should be directed elsewhere.
Professor John Jerrim, lead author of the study, said: "The money the government is planning to spend on grammar school expansion is unlikely to bring benefits for young people.
"Even those children who are likely to fill these new places are unlikely to be happier, more engaged at school or have higher levels of academic achievement by the end of Year 9."
The study looked at results of tests taken in English, mathematics, verbal and non-verbal reasoning and vocabulary, and compared those who attended a grammar school and those who didn’t.
Children were also given a series of questionnaires at the ages of 11 and 14 to gauge their thoughts on mental health, engagement at school, well-being and interaction with peers.
Josh Hillman, director of education at the Nuffield Foundation, which funded the study, said the findings show “the impact of attending grammar schools on a wide range of outcomes, such as young people's self-confidence, academic self-esteem and aspirations for the future.”
He added: "The evidence shows that at age 14, there is no benefit to young people of attending grammar school in these respects.”
Earlier this year, a report said the government should “phase out” grammar schools as they do no better in terms of pupil achievement than non-selective schools and “endanger” social cohesion.
Academics behind the Durham University report argued that increasing selection would be dangerous for equality - and instead the existing 163 grammar schools should be “phased out”.
But this month education secretary Damian Hinds announced £50m of funding for existing grammar schools to expand from this September – which will be dependent on them setting out what action they will take to boost the number of disadvantaged pupils they admit.
Paul Whiteman, general secretary of school leaders’ union NAHT, said: “This is the wrong time to be earmarking an additional £50m a year for grammar school expansion.
“The government cannot point to a single piece of evidence that shows a strong educational benefit of expanding selection at eleven.
“This money should be spent for the benefit of all children, not just the tiny number who attend grammar schools.”
Angela Rayner, Labour’s shadow education secretary, said: “This is simply the latest evidence that the Prime Minister’s pet project of grammar school expansion will fail even on its own terms.
“Additional funding for grammar schools will do nothing to improve the outcomes of the children who attend them, but each new grammar school effectively creates new secondary moderns, a two-tier school system that robs the majority of children of the opportunities that they deserve.”
She added: “Instead of pushing ahead with this discredited and divisive policy, the Conservatives should give every child the support they need, by reversing their cuts to school budgets and giving our teachers the pay rise they deserve.”
A Department for Education spokesperson said: “We know selective schools provide an excellent education – in fact research shows that pupils from disadvantaged backgrounds achieve better results in selective schools, and 98 per cent of them are rated Good or Outstanding.
“We also know these schools are oversubscribed, with parents wanting their children to be able to access this form of education.
“We want more children from all backgrounds to have access to a world-class education, which is why all selective schools applying for funding to expand must not only be Good or Outstanding, but must also make clear how they will increase their intake of disadvantaged pupils and work with local non-selective schools to improve outcomes for pupils of all backgrounds.”
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