Nicky Morgan also urged parents not to take their children out of school next week in a protest over primary testing as she received an icy reception from headteachers today.

School leaders shouted “rubbish” and “you are not listening” as the education secretary addressed the NAHT annual conference on the controversial topics of primary assessment and forced academisation.

But the minister bit back, accusing one headteacher of being "sexist" after he asked whether she controls the department or (the more junior) schools minister Nick Gibb.

Despite the laughter and jeers, school leaders received some reassurance from the education secretary: she insisted that no more than 1 per cent more schools will be below the floor standard than last year – regardless of the fact that tests are “more challenging” this year.

The two-part floor standard is based on attainment and progress - the latter, Ms Morgan said, is where she wants to increase the emphasis.

She said: "As you know, if a school meets the progress standard it is above the floor altogether. We have made sure all who hold schools accountable are aware of this too, and we will continue to do so", she said.

"Historically, the floor standard has identified only a small proportion of schools every year which are below that standard - and this year I can reassure you that no more than 1 per cent more schools will be below the floor standard than last year."

On thousands of parents’ plans to keep their children out of school next week over primary tests, Ms Morgan called the campaign “damaging”.

She said: “Keeping children home - even for a day - is harmful to their education and I think it undermines how hard you as heads are working. I urge those running these campaigns to reconsider their actions.”

But headteachers in Birmingham today were concerned about the teacher assessment of key stage 2 writing – and one school leader claimed it was “discriminatory” against dyslexic pupils.

Simon Kidwell, head of Hartford Manor primary school, asked Ms Morgan to move towards a "best fit" judgement for assessing writing before moderation starts on May 20 - but she said she was "reluctant" to make any more changes.

When Mr Kidwell then challenged the minister on whether she ran the department or Nick Gibb, she said she would not comment on the "sexist remark".

The education secretary also used her appearance at the NAHT conference to reassure heads that "no good rural school" would close as a result of the push towards all-out academies.

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.

Commenting on the speech, Russell Hobby, general secretary of school leaders’ union NAHT, said he welcomed the announcement that the number of schools below the floor will not increase significantly as it was “a welcome assurance that will remove a lot of unnecessary uncertainty for school leaders".

But he added: "However, NAHT would like to see an independent review of the whole primary assessment system, involving school leaders, teachers and parents, all of whom have expressed concerns about the current system.

"This would be the clearest sign that the government is really committed to listening. We cannot endure another year of chaos.”