Bus duty on day one of our new inspection and the pupils are chatting about how it has gone. “What’s all this Ofsted stuff about?” asks one. “Oh I can tell you,” says another who has clearly already been through the process. “It’s dead simple. If you see a bloke wearing a smart suit and carrying a clipboard sitting at the back of your class, you are going to get a bloody good lesson!”

Eighteen years after Ken Clarke set up the system, Ofsted is still the most powerful force in education. What else will bring 120 teachers into work, for two days at least, before 6am?

The displays, even with only two days’ notice, made me think I had walked into the wrong school, while lesson plans and my self-evaluation form were longer than a Victorian novel.

Whatever your role, if you know that Ofsted is going to measure it, you make sure it’s done. The Government wants to encourage diversity, so the specialisms in secondary schools get plenty of attention.

They want the public to believe in the re-branded comprehensive so the Ofsted agenda is to comment on how much the specialism has done to raise standards, support the community ... fill in your own blanks. Schools have had both safety and health scares recently, so plenty of references to diet, lifestyle and safety-conscious staff and pupils go down well.

Racist incidents? Show me your log. Bullying in the playground? Give me the records. If the Government decided it wanted all schools to serve unsalted pistachio nuts on parents’ evenings, test basket weaving skills and dress lollypop ladies in Union Jacks, it would only have to make these things part of the inspection and we would obey.

I experienced my first Ofsted - “a training” inspection in 1992 before the full-scale assault the following year. I am now through number five (just). So what has changed on this latest, tougher framework and over the years?

Short notice has been with us for some time, but I can still remember a ruined Christmas holiday for the leadership team who were sworn to secrecy when we were notified weeks before. We broke the news to staff with a happy New Year greeting - and they still had four weeks’ notice.

The pre-inspection briefing has become as forensic as a murder investigation. Hypotheses are laid out with the misdirected fervour that comes of an ill-spent career salivating over data charts. Crime suspects from the leadership team are wheeled in. “You wilfully, during the months of 2009-10, allowed attendance to drop.” My defence: “Actually, you’ve got the wrong bloke. Isn’t it the parents’ responsibility to get their offspring to school?”

The really big change is that the inspection is now done with you rather than to you. The leadership team double-observed many of the lessons the inspectors went to. We were asked for our grades first to check our judgments. “What did you think of the transitions? What was missing in the questioning? What is your view about the pace?” These were the kind of questions posed to me by the lead HMI as we raced from class to class. This is a great way to influence classroom practice and probably the best professional development anyone could have.

Instead of whispering in separate rooms, their meetings included me and some of the team so the judgments were transparent, too. We were invited to comment but never to canvass: the criteria held firm. The student, parental and staff questionnaires were processed on the day and each section was checked against this data. One interesting student criticism was about the quality of supply. It had never cropped up before but I put it down to “rarely cover” preventing us from using our own experienced and familiar teachers.

The leadership team emailed through the night and from dawn, sharing insights and heading off trouble. “Will we ever sleep again?” mused my deputy at 5am on day two. The final two-hour discussion was all facts rather than opinions, each section scored as if filling up a bingo card.

We followed with absolutely no idea until the end whether we would have that winning “full house”. But it was professional development of the best kind. Even if the number of areas inspected is reduced, as planned, this is a valuable process. Despite popular opinion about Ofsted, I have come to praise it and hope it won’t be buried.

The morning after the judgment, I held up The TES in the staffroom to huge applause because the headline was that outstanding schools no longer faced inspection. I’m not convinced - there is always much to learn and the inspectors surely need to have the widest picture. Still, I’m off to consider academy status now ...

Ray Tarleton, Principal, South Dartmoor Community College, Ashburton, Devon.