Governors at Holland Park school, a top London academy, have launched an independent investigation into allegations about the school’s “toxic” working environment under one of England’s most highly paid headteachers, and the incendiary claim that critical staff questionnaires were hidden from Ofsted inspectors.

Twenty-six of the school’s recently departed teachers have written to the chair of the governing body at Holland Park, whose recent parents include the cabinet minister Michael Gove and the former Commons speaker John Bercow, and separately to the government’s academy regulator, the Education and Skills Funding Agency (ESFA), setting out detailed examples of what they describe as abuses of power by the headteacher, Colin Hall, and his senior team.

The letters, two of which were sent to the governors and copied to the ESFA and a third direct to the ESFA, were sent in June and July. They claim that bullying and intimidation by the leadership at the school, judged “outstanding” by Ofsted, are endemic, affecting staff turnover and morale.

Nine of the signatories had been signed off school for stress, depression or anxiety, which they say was related to the atmosphere at work, with staff being insulted and humiliated, publicly named and shamed for poor work or exam results and given seating plans for staff meetings to indicate who was in or out of favour.

The letters, seen by the Guardian, say that teachers who wanted to leave were subject to intimidating behaviour, such as having classes taken from them. Some say they were threatened with poor references to deter them from attending job interviews or given inaccurate and damaging references that led to some job offers being revoked.

One claims the intimidation did not stop even when she left the school and moved to a new job, as she was told by senior leaders that unless she could provide extra data about a former class at Holland Park, her reference would be changed. “A senior manager at the school subsequently emailed my new school to say they were changing my reference to say I was ‘unprofessional’,” she said.

“Fortunately, my new school understood how unjust and ungrounded this was and so it did not influence my new position. However it caused and still causes significant stress.”

Most of the teachers have asked to remain anonymous, and in the letters state that their experience makes them fearful of reprisals, although some agreed to speak to the Guardian in confidence. Anna King (not her real name) was signed off work by her GP suffering from work-related stress in 2020, after her complaints about the school’s Covid safety measures were unheeded.

“We were asked to sit shoulder to shoulder without masks in staff meetings and it was suggested that committed staff would come to work even if they had possible Covid symptoms. Every other school I knew seemed to be taking it more seriously,” she said.

After she went on sick leave, a senior manager at the school contacted her to suggest that in spite of being unwell, she should provide cover work for some of her classes as she was taking “paid time away from school”.

King wrote to the school’s leadership outlining her concerns. The reply said they did not believe any government Covid guidelines had been breached.

King, an Oxbridge graduate, never went back. “That school is held up as outstanding and one of the best in the country, yet it was a toxic work atmosphere ruled by structures that made people fearful of speaking out,” she said.

Holland Park school has always attracted attention. Called the “socialist Eton” in the 1970s because of its prominent leftwing parents, its reputation dwindled in the 1990s before the arrival of the current headteacher, Colin Hall, who has been credited with transforming it into an outstanding and oversubscribed comprehensive.

The last full Ofsted inspection in 2014 described the school as “exemplary” and said Hall led with “a drive and skill admired by all in the community”. According to the Department for Education’s most recent performance tables, the school’s progress measure is well above average for GCSEs, but below average for A-levels.

Since 2012 Holland Park has been housed in a state-of-the-art building, after part of its previous site was sold off to developers. The new building cost GBP80m – about five times the cost of an average secondary school.

In 2019 it emerged that the headteacher had spent thousands of pounds on Farrow and Ball paint and scented Jo Malone candles. Hall’s salary, at GBP280,000, makes him one of the most highly paid academy leaders in England and, according to a recent analysis, the most highly paid per pupil as the Holland Park Trust contains only one school.

This is not the first attempt to blow the whistle on the culture at the school. An earlier submission made to the ESFA in 2019 from a different group of 30 teachers, raising similar concerns, was followed by an Ofsted monitoring visit to look at leadership and safeguarding in the school, which it then judged “effective”.

It was during this inspection that in spite of the Ofsted guidance stating that staff should be able to complete questionnaires online, this digital option was not possible at Holland Park and teachers who wanted to complete the questionnaire had to do so on paper, to be deposited in an unsealed wicker basket outside the headteacher’s office.

The school retained its outstanding grade but in a letter to the ESFA, one teacher closely involved in the management at the time wrote: “Despite the Ofsted report of January/February 2020 reporting that ‘all members of staff who spoke to us and completed the survey for this inspection were unanimous in expressing how they feel very well supported by senior leaders’, there were a good number of surveys that were critical of the school.

“These surveys were not anonymous or sealed away from the senior leadership team, who read them. Those deemed to be negative in tone or content were then removed from the wicker basket which was later looked at by inspectors.”

A spokesperson for Ofsted confirmed that paper questionnaires had been used because “for technical reasons” online surveys had not been possible and that staff had been able to put these in a collection box or give them direct to inspectors.

She added: “Staff were given every opportunity to speak to the inspection team, as set out in the published report. In addition to staff surveys, this included direct conversations with inspectors as they walked around the school (unaccompanied by senior leaders), and inspectors met with several staff at their request.”

Holland Park’s chair of governors, Anne Marie Carrie, declined to comment on staff turnover and said it would not be appropriate for the school to respond to individual claims but said that the governors took the allegations extremely seriously. “The wellbeing of all members of our school is our highest priority,” she said.

“We were concerned to receive anonymised claims, raised on behalf of former members of the school’s staff. Whilst we believe many of the issues raised have already been considered by our regulatory bodies, it is important that any such claims are thoroughly examined. To provide assurance, we have taken immediate steps to commission an independent external review. This is the appropriate forum for these issues to be explored and for everyone involved to be able to provide detailed, evidence-based responses.”

In a statement via the chair of governors, Hall said he was “saddened and concerned” about the allegations and that he would be supporting the independent review. “We don’t recognise the characterisation of the school or our leadership in the allegations raised,” he said.

Bercow, who was recently elected a parent governor at the school, said he wholeheartedly supported the external review. “This is the correct forum for these issues to be investigated and it is only right that everyone who cares about the school community awaits the findings of that review. I will not therefore comment further on the matter at present.”

The letters’ coordinator, the former Holland Park employee Lara Agnew, said: “We have tried on separate occasions to alert the chair of governors to complaints about the toxic environment, bullying and abuse of power on the part of leadership team and to the way in which the Ofsted questionnaires were handled. So far we have had no response. As a result, we have no option but to involve the ESFA again.”

Several of the teachers, some of whom left in the past year, said they would be prepared to present evidence in confidence to an independent review of employment practices at the school.

A DfE spokesperson said the primary responsibility for academy oversight lay with trustees but said concerns about the school had been raised in 2019, after which the ESFA worked with the school and its trustees on addressing the issues. She added that a similar process would be followed with the more recent concerns raised with the ESFA.