Inspectors have had to apologise nine times to a headteacher after writing a report using data from another school.
The Office for Standards in Education visited Parkside middle school, Bromsgrove, Worcestershire, in November and said it provided satisfactory education but had serious weaknesses in leadership, management, curriculum and assessment.
The education watchdog upheld six complaints made by headteacher John Graham who said that his school was treated unfairly.
Ofsted's 10-page response to Parkside's complaint includes nine apologies and an admission that the inspection report published on Ofsted's website was not of the required quality.
Ofsted found that Michael Merchant, registered inspector, had sent members of his team a pre-inspection commentary containing data relating to another school which appeared to have been cut and pasted into it.
Contractors ISIS have been asked to amend the Parkside report which is available on Ofsted's website.
Mr Graham said: "Inspectors arrived with the wrong impression of the school. It was almost as if the team were trying to justify the incorrect pre-inspection commentary.
"It was not a fair process. I believe that an addendum to the report is not sufficient. The first two pages of the report, should have been re-written."
Other complaints upheld included: the inspectors' failure to recognise improvements made since their last visit; the draft report had not been proof read; and the report failed to recognise a nine percentage point rise in the proportion of teaching judged to be good or better.
A further three complaints were rejected by Ofsted due to lack of evidence.
In April, Banham primary in Norfolk received an apology after chief inspector David Bell described their report as "seriously misleading".
A new inspection framework to be introduced in September 2005 will lead to a sharp reduction in the need for Ofsted to contract out its work.
An Ofsted spokeswoman said: "Ofsted takes very seriously all instances of dissatisfaction with the conduct or outcome of inspection.
"The contractor is currently working with the school to resolve their concerns."
Mr Merchant declined to comment.
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