Media playback is unsupported on your device Media caption Many teachers are angry about plans for mass academisation in England by 2022

Head teachers have threatened industrial action over plans to make all state schools in England academies, after heckling Education Secretary Nicky Morgan at their conference.

National Association of Head Teachers delegates said ministers were simply not listening to their concerns.

Mrs Morgan's speech was met with cries of "rubbish" as she talked about the academies programme and testing.

"I hear the strength of feeling in the hall," she said.

The government has said that all schools will either have to convert to academy status - which sees them funded by the Department for Education but run by a governing body or trust independent from the local authority - by 2020 or commit to doing so by 2022.

What does it mean to be an academy school?

Nottinghamshire head Janice Turner urged delegates at the conference to back a call on the possibility of industrial action.

She said: "For a long time we've been bribed, we've been threatened, we've been blackmailed and we've been punished.

"We have no further means other than to say an absolute 'no - you can't do this to us'."

Head teachers highlighted the opposition to the plans from Labour and some Conservative MPs and councillors, and urged schools to get involved in letter-writing campaigns.

The union also said it would call for a public inquiry into the merits of the academies programme.

Mrs Morgan's appearance at the conference came as primary schools grapple with a new set of national tests for seven and 11-year-olds that are due to take place in the next two weeks.

'A lot of changes'

The education secretary apologised for the accidental publication of the Key Stage 1 Sats spelling paper on a government website, revealed last week, as she addressed the conference.

And she said she understood how much change primary heads had been dealing with this year.

Image caption The benefits of mass academisation of schools are uncertain, says NAHT leader Kim Johnson

"I recognise there have been a lot of changes - there's a new curriculum, a new way of assessment and new tests," she said.

In a question-and-answer session after her speech, Mrs Morgan was asked if she would tweak the wording of the new English assessment, to be taken by half a million 11-year-olds, so that fewer children would be deemed to have failed.

Simon Kidwell, head of Hartford Manor Primary School, Cheshire, called for writing assessments to be judged more leniently to "empower the teachers to make a judgement that will reflect a child's true ability".

Media playback is unsupported on your device Media caption Sally Bates, head of Wadsworth Fields Primary School, and James Bowen, from the NAHT respond to the education secretary's speech

When she said she was not minded to make the changes, Mr Sidwell called out: "Are you in charge of the department or is Nick Gibb?"

Mrs Morgan replied angrily: "I am not going to dignify that sexist question with a response."

This was met with gasps and grumbles from heads gathered in the hall, who had previously been clapping the questioner.

Mr Gibb is the schools minister, and testing and accountability are part of his brief.

Analysis

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By Branwen Jeffreys, BBC News education editor

With an audience of head teachers it was never likely to be a highly rowdy affair.

But with their complete silence as Nicky Morgan walked on stage, low cries of "rubbish" and then loud outraged laughter as she answered questions, head teachers here at the NAHT conference in Birmingham made clear quite how disillusioned they're feeling.

It's not just the plans to force all schools to become academies, which will affect primary schools most, as few have chosen to go down that route.

It's also the changes, delays and uncertainty around primary testing which have left the Department for Education looking ham-fisted.

Read more from Branwen

The education secretary said the tests were a vital part of raising standards.

Heads have been concerned the toughening of Key Stage 2 tests, the results of which are used to hold schools to account, would lead to thousands more schools being labelled failing.

Mrs Morgan said the department would hold the percentage of schools to be deemed failing virtually flat, at only 1% more.

'Widening gap'

NAHT general secretary Russell Hobby told reporters afterwards: "That's very significant. There's a lot of people who will have been thinking that tens of thousands of schools are going to fail."

But there was much vocal anger from delegates who believe standards have been made too hard.

Mr Hobby said he was "worried at the moment about the gap that's widening between the profession and the government".

He said: "Academy status is appropriate for some, but mere conversion doesn't guarantee success.

"What counts is hard work and a clear plan for improvement - both of which can be achieved without conversion."

He said the motion empowered the NAHT to press the government hard on the detail of the plans.

"The secretary of state has said her door is always open and that she will listen. Now she needs to honour her offer."

The NAHT has only taken strike action twice in its history. There was joint action with public sector unions in 2012, and a boycott over Sats tests in 2011.