The government wants to increase the number of schools that select pupils based on performance in an 11- or 13-plus exam to “drive social mobility” and “close the attainment gap” for disadvantaged children.
Education secretary Justine Greening defended the policy at the Association of School and College Leaders’ (ASCL) annual conference earlier this month.
But in a strongly-worded letter published in The Observer today, Conservative former education secretary Nicky Morgan, Liberal Democrat former deputy prime minister Nick Clegg and Labour ex-shadow education minister Lucy Powell opposed the plan.
They argue that creating new grammar schools will do nothing to promote social mobility and warn there is no room for more "division or political ideology" in the education system.
"We must rise to the challenge with a new national mission to boost education and social mobility for all," they write.
"That's why we are putting aside what we disagree on, to come together and to build a cross-party consensus in favour of what works for our children not what sounds good to politicians."
They added that there was an “endless debate” about more selection in the education system that risked undermining positive developments takin place elsewhere.
They wrote: "All the evidence is clear that grammar schools damage social mobility.
"Whilst they can boost attainment for the already highly gifted, they do nothing for the majority of children, who do not attend them. Indeed, in highly selective areas, children not in grammars do worse than their peers in non-selective areas.
"In a time when resources are so limited and many other educational reforms are still in their infancy or yet to be proven - from University Technical Colleges and new T-levels to the expansion of free childcare and hundreds of new free schools - now is not the time for more division or political ideology in education."
Some members of the ASCL heckled Mrs Greening MP and shouted “rubbish” when she defended the Tories’ plans to create more grammar schools last week.
The education secretary told the conference: “We have to recognise that actually for grammars in terms of the disadvantaged children that they have, they really do help them close the attainment gap.
“And at the same time we should recognise that parents also want choice for their children and that those schools are often very over-subscribed.”
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