A grammar school headmaster has questioned the wisdom of a proposal to expand selective education, warning it may force the closure of an existing comprehensive.

Ken Moffat, head of Simon Langton Grammar School for Boys in Canterbury, has expressed concern about Kent County Council’s plans for a ‘satellite’ grammar in Herne Bay or Whitstable.

After Theresa May’s government offered £50m to fund the expansion of existing selective schools, the county council said an annexe to an existing grammar school would be needed to cope with an anticipated rise in the number of schoolchildren living in the area.

But Mr Moffat expressed scepticism about whether there really would be enough demand to fill the schools already in the area, the proposed 1,050-pupil grammar extension, and a new free-school comprehensive that is due to open in Canterbury in 2021.

Writing in Kent Online, he said: “My worry is that opening two new schools in such a relatively short space of time will lead to the closure of one of our current providers.”

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His concerns were echoed by another local head teacher, and seized upon by the campaign group Comprehensive Future, which has said the council’s plan is an attempt to “open a new grammar by the back door” and circumvent the 1998 ban on creating entirely new selective schools.

Dr Nuala Burgess, chair of Comprehensive Future, said: “These school heads know the demographics of Kent: there is every chance of a non-selective school closing.

“If you drain the surrounding non-selective schools of students who are likely to be high achievers, you artificially boost the grammar school.

“Correspondingly, the non-selective school is going to drop for no other reason that they have lost the wonderful mix that tends to produce the best results: every child does better in a mixed-ability environment.

“The impact is quite dramatic.”

Dr Burgess added that a misleading “hard-sell” around the new satellite site, would “seduce” parents into withdrawing their children from comprehensive education when “There is absolutely no need for a grammar school.”

She said: “It is absolutely untrue that a grammar will get those children into better universities, because non-selective schools in this area are actually doing extremely well.

“So if one had to close, it would be a travesty.”

An online petition against the satellite grammar school also questions Kent County Council’s belief that there is a need for more selective school places in the Herne Bay and Whitstable area.

Saying that 50 pupils got into Kent grammar schools last year without having passed an entrance test, the petitioners wrote: “There is no need for 'selective' places locally.

“Four local grammar schools admitted 50 pupils without a test pass. They clearly have a surplus of school places. Kent County Council's predictions of a need are completely wrong.”

In his article, Mr Moffat stressed his continued support for the grammar schools in principle, writing: “Grammar schools can and do bring about social mobility … The old argument of casting children on the scrap heap at the age of 11 no longer holds water. We successfully admit large numbers of students at 11, 14 and 16.”

But he seemed to question the thinking of Barton Court Grammar School, Canterbury, and Queen Elizabeth’s Grammar School, Faversham, which are bidding against each other under the Kent County Council scheme for the right to open a satellite site.

Mr Moffat wrote: “I’m suspicious of schools’ eagerness to expand. It is not the business of schools to start building empires. Running one school alone is a challenge enough and I doubt the ability of school leaders to successfully oversee the running of several schools at the same time.”

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His scepticism about the need for a satellite grammar school was echoed by fellow head Jon Boyes, the principal of Herne Bay High School, a non-selective academy.

Registering his formal objection to the scheme, and raising fears that a non-selective school would be forced to close, Mr Boyes wrote: “The Kent commissioning plan is inaccurate in its projection for need.

“There will not be enough children to fill the schools.

“The plans, should they go ahead, would lead to the closure of at least one existing school.”

He added: “The proposal identifies a high likelihood that two new schools will be opening at the same time, a high school in Canterbury and a grammar in Herne Bay/Whitstable. This will add 300 new year 7 places into a system that currently has spare capacity.

“This will result in the closure of an existing school within Canterbury/coastal area. The consequence of such a proposal would not only be hugely detrimental to the existing provisions but a fundamental misuse of taxpayers’ money.”

Echoing the petition’s suggestion that some grammars were admitting pupils who hadn’t passed the 11-plus, Mr Boyes added: “Within the Thanet, Canterbury and Faversham area there is no capacity issue for grammar provision if grammar schools are already taking children who have not passed the 11-plus exam.

“Therefore, the funding really should be reserved for areas of the country where there is a true shortage of places.”

Roger Gough, Kent County Council’s Cabinet Member for Children, Young People and Education, said: “Kent County Council is committed to ensuring there are enough suitable school places for all children and young people in Kent in the coming years.

“The latest version of the council’s Commissioning Plan for Education states that if new housing is delivered as expected over the next few years there will be a gradual need for more grammar school places in the Canterbury and Faversham area, which includes Herne Bay and Whitstable. The need for Year 7 places is expected to increase from 37 in 2019 to 133 by 2023.

“The preferred option for meeting this need would be to establish grammar provision in one of the coastal towns, in the form of a satellite of one of the existing grammar schools in the area.”