Grammar schools in England may ask parents for hundreds of pounds a year to cope with funding cuts, their headteachers’ association has warned.
A majority of grammars will be left worse off by proposed funding changes, according to analysis by the Grammar School Head’s Association.
A number of Conservative MPs are urging the government to change its plans.
But the Department for Education said it was ending a postcode lottery in school funding.
The new system is designed to support deprived areas by reallocating existing funding.
But the Grammar School Head’s Association has said that while 60 grammar schools will gain, 103 are set to lose money, 62 of which will receive cuts as deep as any in England.
The head of Altrincham Grammar School for Boys, Tim Gartside, told BBC Radio 4’s Today programme that he and his governors were considering asking for voluntary contributions of £30 or £40 pounds a month from parents if the cuts took place.
Mr Gartside, who also speaks for the Grammar School Head’s Association, said “many other” grammars were considering a similar move.
“What we’re looking at here is funding which is fundamentally going to change the nature of grammar schools,” he said.
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Some grammars already use parental contributions to help cover spending.
Southend High School has folded parental contributions into its main accounts for two years and is increasingly using it to contribute to spending on salaries and library books.
Latymer School in north London has asked parents for contributions to help cover a funding shortfall, and expects more schools to do likewise.
Schools are not allowed to charge for education in school hours, but no law prevents them from asking for voluntary contributions for the benefit of the school or any school activities.
The existing funding system for grammar schools was criticised for being arbitrary and tending to give more money to city schools than those in suburbs and shire counties.
The co-chair of the F40 campaign for fairer funding, Conservative MP Alex Chalk, said there was concern about the risk of replacing one geographical injustice simply with another.
The campaign spent years pushing for funding reform.
He welcomed the fact the funding rules were being rewritten, but said: “The bottom line is that it’s created some distorted outcomes which we think require some significant re-modelling.”
Ministers face pressure from other Conservative MPs concerned about proposed cuts affecting schools of all kinds in their constituencies.
Some of them privately expect the government to change its plans.
A number of Tories are backing heads’ demands for a minimum per pupil funding guarantee that teachers and MPs proposed to the schools minister Nick Gibb in a meeting at the start of the year.
One head who attended the meeting – Sarah Burns of the Sandbach Boys School free school, Cheshire – said she would have to consider running her school on a four-day week, scrapping the sixth form or cutting arts subjects from the syllabus.
She said: “They are absolutely realistic possibilities given the level of cuts and the fact that we’ve already cut to the bone.
“Given those levels of cuts we will have to take some really hard decisions like those.”
A spokesman for the Department for Education said: “We are going to end the historic postcode lottery in school funding.
“Under the proposed national schools funding formula, more than half of England’s schools will receive a cash boost in 2018-19.”
A consultation on the proposed new funding formula is to run until March.