The National Association of Head Teachers (NAHT) has voted to “vigorously reject” the expansion of grammar schools in England.
Headteachers at the union’s conference voted at 97 per centre in favour of rejecting Conservative plans for a new wave of selection schools in the absence of "any compelling evidence that it promotes social mobility".
The NAHT argued early years education was a bigger driver of social mobility and should be the focus of the next Government. It called for more funding to ensure all children were given the same chances early on in their education.
Theresa May made the expansion of grammar schools the cornerstone of her social mobility programme but the move is opposed by Labour and the Liberal Democrats.
The union also used the conference to highlight the fact secondary schools in England were facing a “perfect storm” of pressures, including £3bn real term cuts to school budgets by 2020, changes to exams, difficulties in recruiting teachers and Brexit. It added these pressure could have severe consequences for children.
NAHT delegates backed a motion for executives to keep school funding as a priority and "explore every available option open to schools" if "irresponsible funding cuts" are not reversed.
This could include school leaders refusing to submit their budgets or running a deficit budget and headteachers introducing a four-and-a-half day week as “a last resort”.
#NAHTconf Motion 8: vigorously reject expansion of selection; grammars do not improve social inequality, early years edu does. Passed 97%— NAHT (@NAHTnews) April 29, 2017
Carlisle headteacher Graham Frost said no school leader wanted to cut their curriculum or staff numbers but that headteachers already "trimmed our sails".
Speaking ahead of the vote in Telford, NAHT general secretary Russell Hobby said there was a risk the secondary school system would break.
He said: "The combination of challenges facing secondary schools and their students has never been greater.
"Many school leaders are concerned about maintaining high standards in the face of simultaneous upheaval on so many fronts. It's a perfect storm.
"The Government is loading more uncertainty onto the secondary system than ever before. There is a real risk it will break."
Mr Hobby repeated warnings schools were facing "unacceptable levels of financial pressure” but that the government was not responding to headteachers’ concerns.
More than 500 head teachers signed a letter to Theresa May on Saturday accusing her of pushing Britain’s education system to breaking point.
In the letter sent by Mr Hobby and published in The Mirror, teachers said they were having to ask parents for donations in a desperate bid to pay staff and fund resources.
Mr Hobby told the conference school funding had to be among the questions raised within each constituency ahead of the 8 June general election, the BBC reports.
“All that's left is for us to make it a general election issue. We need to make sure that all our parliamentary constituencies are asking questions on school funding,” he said.
Last month, the powerful House of Commons Public Accounts Committee warned that the Department for Education had “buried its head in the sand” over the problems the funding cut will cause.
The Department for Education previously said schools funding is at record levels and is set to rise further.