A second head teachers' union has called for an immediate halt to graded Ofsted inspections in England and raised the prospect of strike action, saying the new grading system has become an "overbearing mess".

The Association of School and College Leaders (ASCL) said it had passed a motion of no confidence in "the ability of Ofsted's leadership to address the damage being done by the new inspection system".

It said it was "increasingly concerned about the welfare and morale" of its members, and warned it would hold an informal ballot on industrial action if nothing changed.

Ofsted: 'on the right track'

On the same day, Ofsted's chief inspector, Sir Martyn Oliver, said an independent survey of education providers showed the inspectorate was "on the right track".

"I know that workload and wellbeing in education continue to be challenging, and I don't take that lightly," he said. "We will keep listening to feedback from professionals and parents alike as we continue to improve what we do."

An Ofsted spokesperson said it had "worked hard to reduce the pressure of inspection in recent years" and was working with the Department for Education on workload and wellbeing.

Two unions, one message

ASCL's move follows a similar call from the National Association of Head Teachers (NAHT), which said the changes made to inspections just under a year ago had been severely harmful to leaders' welfare.

Last month the NAHT said testimonies from members suggested some heads had faced "intolerable" pressure, leaving them "overwhelmed" and "anxious". It warned the system could pose a "risk to life" if changes were not made.

The legacy of Ruth Perry

Ofsted introduced the report card system last November to replace its one- or two-word judgements, such as "outstanding" and "inadequate".

The change followed the death of head teacher Ruth Perry, who took her own life after Ofsted downgraded her school in Reading from "outstanding" to "inadequate". An inquest concluded that the inspection had contributed to her death.

The new report cards grade schools on several separate areas rather than giving a single overall verdict. Ofsted argued this would give parents a fuller picture and reduce the pressure of a single high-stakes label.

Heads' unions say the opposite has happened: with more areas graded, there is more to prepare for and more ways to fail.

What the survey found

An independent interim report commissioned by Ofsted, based on focus groups and nearly 700 responses from school, college and early years leaders, concluded this week that leaders were "generally positive" about inspection.

It found the process was "more collaborative and accounts for context better than previous inspections".

But it also found that nearly half of leaders who responded — 48% — said their staff's mental wellbeing had been negatively affected "a fair amount" or "a lot" by a recent inspection. A full report is due next year.

Government response

The Department for Education said ministers were "working with Ofsted and school leaders to ensure accountability is fair, proportionate and focused on helping every school get better".

It added that inspection "must reflect the reality of what is happening in schools, recognise the context they are working in, and support improvement rather than add to the pressures they already face".

Teaching unions have also warned that recruitment and retention of head teachers is suffering, with surveys suggesting many leaders are considering leaving the profession early because of workload and accountability pressures.

Parents' groups are divided. Some value clear grades when choosing a school, while others say a fuller written picture is more useful than a label.

With two major unions now united against graded inspections, ministers face a difficult choice between defending a system introduced to reduce harm and reopening the question of whether schools should be graded at all.