Getty ImagesA school leaders' union says it is taking legal action against Ofsted over proposed changes to the way it inspects schools in England.
The National Association of Head Teachers (NAHT) has asked for a judge to review plans for new school report cards, which are due to be introduced in the autumn after Ofsted got rid of its old one or two-word judgements.
Its general secretary, Paul Whiteman, said the plans amounted to "another crude grading system to hang over the heads of leaders", and should be scrapped.
Ofsted said supporting mental health was an important part of the proposals, and that the union's legal claims were "plainly untenable".
The NAHT said it had issued a claim with the High Court for a judicial review on Saturday, as school leaders gathered for its national conference in Harrogate.
The decision comes after the sister of head teacher Ruth Perry, who took her own life after an Ofsted inspection, called for the new system to be delayed.
A letter signed by Prof Julia Waters with more than 30 others last week, including the general secretaries of four teaching unions, warned the changes would "continue to have a detrimental impact on the wellbeing of education staff and hence on students' school experience".
On Friday, Mr Whiteman told NAHT members: "Following Ruth's death, we were promised change.
"What we've been offered, if you scratch beneath the surface, is more of the same - another crude grading system to hang over the heads of leaders."
BBC / Kate McGoughMr Whiteman said school leaders embraced accountability, but said it was "utter nonsense" that Ofsted's proposed model was the only viable way to assess schools.
He called on Ofsted boss Sir Martyn Oliver to "scrap these proposals" and "think again".
A debate by NAHT representatives on the proposed changes was scheduled to take place on Saturday, the second day of the union's conference.
Ofsted's report cards are set to assess key aspects of each school, including quality of education, behaviour and attendance, personal development, and leadership and management.
There will be five possible grades for each area:
A separate part of the report card will say whether the school's duties around safeguarding have been met.
Education Secretary Bridget Phillipson has said the new system will provide "rich, granular insight" to help deliver "tailored" support for struggling schools.
Ofsted's consultation on the changes ended this week.
An Ofsted spokesperson said: "Supporting the mental health of those we inspect is an important part of the development of our proposals and we are already hearing positive feedback through our inspection tests.
"Our legal team have responded robustly to NAHT. Their suggested claims are plainly untenable. If legal action is launched, we will resist and seek costs."
