Disgruntled headteachers could take industrial action after they voted overwhelmingly against schools having academy status thrust upon them.

A motion that "no schools should be forced to become an academy" was amended to include the "last resort" move at the National Association of Head Teachers (NAHT) annual conference in Birmingham. It was passed by 95 per cent of delegates.

The alteration instructed the association's national executive to "consider all options open to NAHT up to and including as a last resort industrial action to ensure the defence of our comprehensive state education system".

Education secretary Nicky Morgan told the conference on Saturday morning, ahead of the motion, that she would go ahead with her reform plans because she believed academies "make it easier to spread the reach of the best leaders over several schools".

She tried to alleviate the headteachers' concerns about the proposals by telling the conference that "no good rural school will close as a result of this policy".

The controversial plan, considered the greatest shake-up to the education system in generations, would force thousands of schools to become academies over the next six years.

Ms Morgan said she foresaw that most academies would work in "local clusters", enabling teachers to extend their reach locally, in order to support other schools to succeed.

Describing academies as the "right step" for the education system, Ms Morgan said: "The autonomy academy status brings means putting power into the hands of school leaders, because we improve outcomes for young people by ensuring the teachers who teach them, and the heads who lead their schools, are given the freedom to make the right decisions in the interests of those children."

She admitted that academy status alone does not raise standards but said it was the "framework of collaboration and support it provides that does". Ms Morgan said it was not about "creating a system of survival of the fittest" but rather building the "scaffolding" to provide swift action in struggling schools.

"Academies make it easier to spread the reach of the best leaders over several schools; recruit, train, develop and deploy better teachers, incentivising them to stay in the profession through new career opportunities; and ensure teachers can share best practice on what works in the classroom," she said.

But her speech did little to lessen opposition to the proposals, as delegates at the conference made a last minute amendment to a motion to include industrial action.

Members' passing of the motion has put the chance of industrial action on the table.

The motion also commits the executive to "ensure that school leaders are supported in resisting forced academisation" and to "continue to challenge the perception that structural change is a panacea for improvement".

Russell Hobby, general secretary of the NAHT, said school leaders had "huge concerns" about the academy plans and there were also issues with testing.

He told the BBC: "I think the gap between the profession and the Government has never been wider than it is at the moment.

"We don't only have assessment, we also have an enormous number of mistakes, delays and confusions around testing at the primary level as well and to be frank I think that's making people more angry than academies at this point in time.

"A Government that talks about professional autonomy and delegating the control to the front line really needs to listen to that.

"They have not yet heard a case to convince them of the merits of converting every school to an academy. What they want to do is focus on teaching and learning in the classroom, not on all the logistics, admin and legal changes that will be a distraction for them," he said.

Under the academy plans, 17,000 state-run schools in England will become autonomous academies run by trusts before 2022.

The move, outlined in a White Paper last month, is designed to improve education for children but has prompted a backlash.

Two petitions against forcing schools to become academies have been launched - attracting around 300,000 signatures - while Ms Morgan was heckled at the conference again on Saturday.

Not all headteachers present at the conference clapped when Ms Morgan finished her speech and several interrupted her as she spoke.

At one point, the Education Secretary told the headteachers that her "door is always open", sparking jeers and laughter from delegates.

The president of the NAHT, Kim Johnson, said small schools would "surely suffer" under the radical academy changes.

In a letter to the Daily Telegraph he called on Ms Morgan to be clear on how the plans will benefit children rather than be an "administrative convenience" to the government.

Mr Johnson wrote: "I will happily persuade my colleagues of the merits of autonomy and freedom. But the cost of the current initiative to turn every school into an academy is high and the benefits uncertain. Smaller schools will surely suffer."

Ms Morgan appeared before the Commons education committee on Wednesday where she acknowledged the considerable weight of opposition to the education White Paper, but said she had received plenty of support in private.

She told the committee: "We haven't had the pitchforks yet, but perhaps I should look outside the committee room when I leave."

Ms Morgan said she was fully committed to the plan and denied claims academy status was being "inflicted" on schools, although it will be compulsory if voted through parliament.

The Department for Education said it was "disappointing" to hear of a union considering industrial action which it said "holds back children's education, disrupts parents' lives and ultimately damages the reputation of the profession".

A spokeswoman said: "The academies programme is at the heart of our reforms which have raised standards for children across the country, including many schools that stagnated under local authority control - 1.4 million more children are in good or outstanding schools compared to 2010.

"We want to work constructively with the sector to ensure every child has the excellent education they deserve."

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