A Sheffield secondary school was visited twice in rare move by Ofsted after claims an initial inspection was 'flawed'.
Forge Valley School, in Stannington, has been judged as requires improvement after Ofsted inspectors visited in April and then took the unusual move to visit again in June.
The rating is an improvement on the school's last inspection in 2013, which placed it in special measures, but headteacher Dale Barrowclough said he has 'well founded concerns' regarding the inspection process and that the final report is based on an 'incomplete, partial and flawed' inspection.
Sources close to the school, which has been hit by three tragedies this academic year including the death of a teacher, said inspectors lost vital safeguarding documents and breached its own guidelines by looking at pupils' predicted progress 8 scores - which judges progress through secondary school..
Mr Barrowclough said it was apparent that the initial inspection was 'insecure and incomplete'.
Ofsted said it was investigating the circumstances which led to inspectors having to pay a second visit to 'gather additional evidence'.
Mr Barrowclough said he was concerned about the validity of any evidence gathered during the first visit and its impact on the final report.
He added: "It is demonstrably apparent that the initial inspection was insecure and incomplete resulting in Ofsted having to pay a further visit to the school to secure the inspection evidence base.
"It is also my view, and that of staff that some inspectors did not conduct their business, in a manner as outlined in their code of conduct, 'without fear or favour'."
Mr Barrowclough said that although he is pleased the report positively reflects the school's vast improvements, he felt it focused on the 'minority rather than the majority to justify a less than good grade.'
He said the school was 'disheartened' about the 'enormous impact' that a very small number of low-attending, disadvantaged pupils have had on the final judgement.
"Despite unrelenting and recognised efforts by the school to encourage this group of pupils to attend regularly, under the current Ofsted framework, schools are held to account for such factors that are often beyond their control," he said.
"However, at the same time, Forge Valley has improved the progress of disadvantaged pupils, who attend regularly, to the extent that they now outperform other pupils. It is regrettable that this is not an overt message in the report.
"Both teaching and behaviour are much improved at Forge Valley and we continue to drive further improvements.
"It is disheartening to read in the report comments focusing on the minority rather than the majority to justify a less than good grade and not reach a conclusion based on typicality.
"It may be the case that our Progress 8 scores - fall this year. This will be a result of increasing numbers of pupils who are ill or dealing with tragedy.
"This inspection and its report have told us nothing about our school that we did not know already.
"The process has added unnecessary work and pressure, leaving us with a judgement that is based on a demonstrably incomplete, partial and flawed inspection, under a framework that affords inspectors the discretion to label schools based on factors often beyond the influence of school leaders."
The school's sixth form provision was judged as good and inspectors also praised Mr Barrowclough and senior leaders for improving the school since it became part of Tapton Academy Trust.
The report said that under 'determined and decisive leadership' and 'despite many obstacles to overcome,' the school has improved.
It praised the positive relationship between teachers and pupils and the 'constructive and purposeful atmosphere' created by staff through praise and humour.
The school, part of the Tapton School Academy Trust, has been hit by three tragedies this academic year.
Teacher Lynsey Haycock died in September 2016 after breaking her leg in a classroom. In December, 15-year-old student Hakeem Pickering-Smith sustained life-changing injuries after being hit by a car, and in March, 14-year-old Scott Marsden died after collapsing during a kickboxing contest
Mr Barrowclough said: "I want to thank every member of staff at Forge Valley.
"They are the ones that have worked unrelentingly to deliver the overt improvements at the school.
"They are the ones who are asked to account for what is often impossible. They are the ones that have provided the care, support and compassion to pupils and their families during three tragedies which beset the school this year. They are the true heroes of the school.
"If Ofsted want to conduct their business in such a way that we have experienced and stick a label on us; that is for them.
"It will not however dampen our determination and love for our school and our deep and unflinching commitment to support every Forge Valley student to be the very best that they can be."
An Ofsted spokesman said it was 'rare' to visit a school twice and is investigating the circumstances which lead to it.
He added: "Ofsted visited Forge Valley School a second time to collect additional evidence, in line with quality assurance procedures.
"Following this additional inspection day, a review of all the evidence confirmed the judgements made in the recently published inspection report.
"We are taking action to address the circumstances that required us to return to collect additional evidence.
"We are also investigating concerns raised by the headteacher has raised."