As an Ofsted inspector, as well as a headteacher, Richard Glasby has passed judgement on numerous schools. But following the "galling" experience of seeing his own school graded "satisfactory", he is quitting the inspectorate.
Mr Glasby, who worked part-time for the watchdog for three years, insists that he is not walking away because of "sour grapes", but because of fears that Ofsted "doesn't know what it is talking about".
When inspectors visited Hassenbrook Academy in Stanford-le-Hope, Essex, Mr Glasby is convinced that they had made their minds up before seeing the school. It was the third consecutive inspection that had resulted in a "satisfactory" rating.
"I felt as if I was talking to a brick wall," Mr Glasby said. "It felt as though the inspector had made his mind up before he walked in the door. We might as well have packed up and gone home at 9.30am on the first day."
The school also feels it has been harshly treated despite its success in increasing pupils' attainment.
"We have a school where, every year, the intake in key stage 2 has significantly below average results," said Mr Glasby. "In the past four years, we have been above the national average for GCSEs. How is that satisfactory? I don't get it. How far above the national average do you need to be?"
Following the June inspection, Ofsted reported that Hassenbrook still had much work to do. "Despite improvements in teaching," the report said, "it is not yet a good school because teaching is inconsistent between and within subjects, students' oral and written skills are not sufficiently strong and middle leaders are not involved enough in driving forward whole-school developments."
This assessment is not shared by Mr Glasby, who has served on about 15 inspection teams. "I thought we deserved good. It's pretty galling. I don't think it's a fair reflection of my staff and their work.
"There's a considerable degree of unity among staff and the feedback from parents, saying Ofsted don't know what they are talking about."
Despite Mr Glasby detailing his concerns about the conduct of the inspection team in a 38-point letter to Ofsted, the watchdog dismissed all but one of his complaints.
"I am unhappy about a lot of things to do with the inspection," he said. "I'm pretty sure it's not just sour grapes; if I was the inspector carrying out the report, I don't think I would have been happy either.
"I went into inspection because I wanted to learn how it works, thinking it would be useful in my school and that I could bring things back that I had learned. But I'm quite disillusioned with Ofsted.
"There are so many things that are vague in the framework. Professional judgement is involved, but there are so many variations in how people see similar situations. There's a lack of consistency."
As a result, Mr Glasby has decided to stop offering his services as an additional inspector. He told TES that he expects Ofsted's new framework, introduced last month, to throw up even more problems.
"I think it's more inconsistent now than it was under the last framework," he added. "I don't think Ofsted will ever admit to these problems."
Brian Lightman, general secretary of the Association of School and College Leaders, said he was "increasingly concerned" about his members' feedback from inspections, and feared the situation could worsen under the new framework. "It is absolutely essential that schools get a fair hearing," he said.
Ofsted declined to comment.