Recently we had our OFSTED inspection under the new framework. So, we were one of the first schools to experience the new system which grades teachers, and reports to the head those with the lowest and highest gradings.

I wasn’t opposed to this idea, especially since I’ve always felt that weak teachers shouldn’t be allowed to continue and that for the rest - the vast majority - who are good teachers, something radical was needed to confirm their professionalism and help them take pride in their work. Sadly, our experience has changed my mind.

Our inspection showed me that the new procedure is far too blunt an instrument to achieve either my objectives or indeed the reported objectives of Chris Woodhead and the government. I have no criticism of our inspection team who were thoroughly professional and made sound judgments. Nor is this written out of a sense of bitterness - 15 per cent of our staff were reported to the principal for their high quality and none for poor teaching.

The problem is that the inspection does not allow inspectors to see enough lessons to make a reliable judgment. There are so many other factors to consider in covering different age and ability groups during an inspection and it is inevitable that over the course of five days or so, not all teachers will be observed on an equal number of occasions. Even if they were, how reliable is one week?

Inspection contracts are dependent on the size of school but in large schools there is a presumption of economy of scale. To report a teacher to the head, he or she has to be observed on three occasions; this is virtually impossible for all teachers and can be achieved only by seeing part lessons. No inspector would make an extreme judgment based on 15 minutes’ observation. If the first 10 minutes are not observed, one cannot make a reliable judgment on key aspects of the lesson.

I am happy for the success of those teachers who were reported for excellent teaching but I can think of at least six others who are at least as good but were not reported. In some cases, they simply were not seen three times; in others, the first observation was of a revision lesson. Such a lesson is necessary and precisely what pupils need in the run up to examinations. However, a revision lesson is unlikely to attract a grade 1 or 2.

It is proper that classroom observation is the basis for school inspections. But when a grading system that was designed to allow a judgment to be made on the overall quality of a school is used to assess individuals, there are bound to be problems. It assumes that all the aspects important to promoting pupil progress are there to assess within the lesson.

But good teaching requires more than that; it is about sustained monitoring over time, effective reporting to pupils and parents and so on. Furthermore, pupil progress is dependent on a number of other factors in the school.

What about the head of department, for example, who takes the most difficult students into his or her group? By doing this, the children’s progress is secured but the head of department misses out on a commendation.

Post-OFSTED there are bound to be staff who feel aggrieved and under-valued. They will say nothing and will continue to provide excellent teaching because they are utterly professional. But will they be disadvantaged in future career moves?

I can see the advertisement now: “Only teachers with an OFSTED lesson grading of 1 or 2 need apply.” And where there are weak teachers, will governing bodies be able to enforce dismissal against the defence that inspection did not identify them as 6 or 7?

The only way to identify strengths and weaknesses in teachers is to develop a rigorous appraisal system that is based on a nationally agreed set of performance criteria.

Such a system should include other aspects of a teacher’s work which are necessary to being effective. A framework of criteria based on classroom practice and annually assessed within school would not only be more rigorous, it would give teachers a clearer sense of what is expected. I believe the Teacher Training Agency is working on this area now. These annual statements could be open to the scrutiny of OFSTED inspectors as part of their work. Poor performance could trigger visits from such an arbiter to check the school’s judgment.

It is not just the divisiveness and injustice of the new grading system that is wrong, it is the inaccuracy: the criteria are not explicit and there appears to be no system of standardising judgments. I suspect the tendency will be for inspectors to err on the side of caution. Fewer teachers than deserve it will be graded 1 or 2 because inspectors will wish to be absolutely certain it is warranted. And even fewer will be graded as 6 or 7 because no inspection team will want to be the first in court justifying their judgment.

Kevin O’Regan is vice principal of Banbury School, Oxfordshire