Of all the educational cliches, the one I find most ludicrous is trotted out each year when exam league tables are announced. No headteacher, LEA chief, politician or academic expert can begin a discussion about what the results mean without saying: “First, I would like to congratulate all our young people and teachers who have worked so hard to achieve this year’s good results. We must all acknowledge the exceptional effort, expert teaching and commitment that lie behind this wonderful achievement that has ensured standards have risen yet again.”

It’s absurd. Everyone knows it isn’t true. Not all the young people have worked hard. Neither have all the teachers. The correlation between conscientiousness and success in exam results is extreme. We can all think of pupils who get a C through dogged determination and those who get the same grade with little effort. And in any staffroom there are many who give every minute of their time to running extra classes, seeing individual children with difficulties, setting targeted work, only to see the pupils of colleagues wedded to tick-and-cross marking walk away with the A grades.

But for some reason we pretend this is not so. It’s all part of the smokescreen around what national exam results really mean. Do they have any meaning at all?

Successive governments are desperate to make those inside, and more so those outside, education believe they are solely what schools are about. Given the dominance exams have over the curriculum, and the judgments made on individual children, teachers and schools, we may all hope they have some value.

But wherever you start to look, the waters are muddy. League tables, we are told by politicians, are to better inform the public. Schools must be made accountable. In the press, where most results are shown, league tables are muddling. Some papers publish a total point score, some publish scores based on just the top grades, some do lists according to the quantity of children, some ignore schools that only have small exam entries, some ignore particular exams altogether.

Ucas has now brokered a universal tariff which will be fully operational next year. Everything can be converted into a number and added up. The balance between the scores given to high grades and low grades is changing, so high grades are worth less. Of course politicians say standards are rising. But ask employers, university tutors, anyone who has taught for more than a few years, and they will say exams are easier than they were.

My school provides a small example. We have entrance papers and results going back many years and it is possible to tell that the range and ability of girls coming to us at 11 was similar 20 years ago to what it is now. The percentage of those achieving A grades, however, has risen by about 20 per cent.

Exams have always been a rough and inaccurate tool. What is new is that it has become politically incorrect to dare to say so. We have created a brotherhood of educationists whose business it is to prove a blunt implement is a laser beam. Very 1984.

Sarah Evans Sarah Evans is head of King Edward VI high school for girls, Birmingham