Teacher Lynne Fox received a prestigious Pearson national headteacher of the year award last summer. Judges praised her as a “tour de force” who had transformed her comprehensive, Bramhall high school, overcoming significant challenges such as funding cuts and staff redundancies to secure “the very best educational experience for students”. The school, they said, was “a vibrant environment where enthusiasm and creativity are encouraged”, leading to “outstanding” results.

A few weeks later, Fox found herself leaving a meeting with Ofsted inspectors in tears. “It was the only time in my professional career as a leader that I’ve ever felt that bad. That low.”

The watchdog told her Bramhall high – currently the second-highest performing school in Stockport, despite suffering budget cuts of £400,000 over the past three years – did not provide a “good quality education” and “requires improvement”.

Fox, 58, who has calmly weathered 12 Ofsted inspections in her 17 years as a headteacher, was shaken to the core. “For three years running, when GCSEs have got more difficult nationally, our results have bucked the trend: we have gone up and up in our outcomes. It just doesn’t correlate. It’s so unjust.”

A month later, Ofsted then sent her a different, more factual report summarising the school’s performance. It stated the school was in the top 20% nationally for almost all of its subjects, and highlighted the “significantly above average” progress and outcomes students achieve and the “strong academic foundation” of the school’s curriculum. Yet it was the school’s curriculum Ofsted lambasted in its inspection report last term. “The only thing that’s consistent about Ofsted is that it will be inconsistent,” says Fox. “It’s ludicrous.”

In September, the watchdog overhauled its entire inspection framework, announcing it was shifting its focus on to “the real substance of education: the curriculum”. Now, even when a school is rated “good” by the inspector in all areas other than “quality of education”, it can be given a rating of “requires improvement”. This is exactly what happened in the case of Bramhall high.

Inspectors found fault with the school’s decision to start teaching pupils their GCSEs in year 9, a year earlier than usual, and criticised the breadth of education provided as a result. They also disparaged the school’s support for its most disadvantaged pupils and those with special educational needs, despite improvements in the progress of these students. “We have significantly narrowed the [attainment] gap for both those groups,” says Fox. “I will never be satisfied until we keep moving those figures up. But I feel it is unfair the report didn’t recognise the narrowing of that gap, and the work that has had to happen, for that to take place.”

The government recently announced plans to increase Ofsted’s powers to carry out school inspections. Longer, more detailed inspection visits are now in the offing, along with more frequent inspections for outstanding schools. By contrast, Labour’s manifesto had pledged to scrap Ofsted entirely, amid reports of teachers experiencing record levels of stress.

Since the judgment was handed down, Fox has needed extra support from friends and family and has had sleepless nights and recurring feelings of bitterness and frustration. “I’ve never felt so powerless as I do now.”

Bramhall high school in Stockport. Photograph: Gary Calton/The Observer

Challenging the report did not help. In response, she says, Ofsted inspected its own inspection and upheld its original rating. Fox then asked the watchdog to share evidence about how it had reached its “astounding conclusions”. But she was told just before Christmas such a disclosure would “harm potential future inspection activities” and was not in the public interest.

This proved the last straw and she announced to staff and parents last week she was taking early retirement and would be leaving in the summer. “I feel totally disillusioned and angry. That isn’t healthy for me – and it’s not in the best interests of the school.”

After the report was published, more than 300 parents wrote to Ofsted praising the school and its impact on their children, she says. “All our data shows a three-year improving trend. That should have been recognised. Our teaching is of high quality, our outcomes are brilliant, our kids are engaged and our staff are hard-working.”

She is scathing of Ofsted’s autocratic approach: “This process has not provided us with anything that will bring about school improvement. It’s just caused disillusionment across the school.”

An Ofsted spokesperson said: “We are saddened that the headteacher has made the decision to resign, especially as our inspection team judged the school’s leadership and management as good. Beyond this the inspection report explicitly commented on the good job that she and her senior staff had done in improving the school. The report also recognised the school’s improvements in the pupils’ behaviour, personal development and GCSE results, while pointing out the improvements needed to the quality of education. However, we do understand the strength of feeling among the school community about the head’s departure.”

The watchdog added that while it does not comment on individual complaints about Ofsted inspections, it treated them all seriously and investigates them thoroughly. “As an accountability body we recognise the value of external scrutiny and we report on the handling of complaints each year in our Annual Report and Accounts. This shows that, where appropriate, Ofsted takes prompt steps to put things right when a complaint is upheld. If a school requests an internal review it is considered by a panel that normally includes an external representative who is not employed by Ofsted but is, for example, a school leader. If a school is still not happy then it can complain to the Independent Complaints Adjudication Service for Ofsted, which reviews the way that we deal with complaints. And, more broadly, Parliament scrutinises our work.”For Fox and her staff, these words will bring little comfort as they await the next gruelling visit from Ofsted’s inspectors, which could take place at any time. “I have no faith left in the system,” says Fox. “You can’t win. Ofsted is a machine you can’t beat.”