The final week for changing estimated grades has come round again,” said our school’s examinations officer; as with many of the school’s early morning announcements, we all noted it in silence before moving on to the day’s hurly-burly.

It is an annual ritual, familiar to all. The mark sheets appear in our pigeonholes, we fill them in as quickly as requested and, for the most part, forget them. In reality, however, requests for estimates raise the question of professional judgment, and have a potential bearing on thousands of young lives.

The seriousness of what we are asked to do on these occasions ought to be painfully obvious. After all, out there in the real world, the word “estimate” presages nightmares. Whether you are building a small extension to your house or the Government is commissioning a hi-tech jet fighter, the final cost will inevitably be 20 to 2,000 per cent more than you were told.

We estimate our students’ grades and, even if we do so with the aid of the proper records and data, once done they are someone else’s concern.

But will their use be in proportion to our intention, or will they be charged with up to 2,000 per cent more significance than we imagined?

When you are asked for an estimated grade, what criteria do you apply? Do you write down the grade a student’s previous performance would indicate? If so, what sort of activities and tests generated them? Are they comparable with, say, six weeks of GCSEs? You probably thought of that and compensated.

Well, how about their coursework? Are your estimates influenced by the marks you have already submitted? Did you state that the student would obtain a B on the basis that he or she achieved that level in units of work produced over several weeks with heaven knows how much redrafting assistance? But of course, you took that into account.

So what do you think “estimated” means in this context? And have you noticed that what we are told are “estimated” grades are sometimes referred to as “grade forecasts”? (Look carefully at the top of each column on the official forms.) To my way of thinking, those are two very different things. “Estimated” grades suggests a figure or letter that might almost as easily be the adjacent one. I will not be too disappointed if a student I “estimated” as a B gets only a C, nor will an A unduly surprise me. But a “forecast”? Well, that suggests I am bringing to bear a fair whack of my expertise and experience, and if the student doesn’t get the B I predicted, I will be as wrong as weatherman Michael Fish’s legendary “there will be no hurricane” forecast.

But surely, I hear you say, it’s a matter of common sense. An estimated grade has to be a best-case scenario, doesn’t it? No one criterion can cover everything, so you take everything into account, don’t you? Including: whether the student will bother to turn up on the day anyway or be on a cheap holiday in Florida, at one end of the scale; to minute but observable variations between performance in those key stage 3 SATs papers you dissected thoroughly two summers ago and which you still keep to hand for weekly research and target-setting purposes. In other words, as a typical teacher you do a thorough job and your grade estimates represent as good a reflection of a student’s potential as anyone could possibly produce.

But how are they used? Have you ever tried imagining what is done with your carefully considered assessment input? Does your imagination conjure up a picture of a bespectacled, Dickensian clerk, surrounded by ledgers, spending the summer scrupulously going through every teacher assessment and factoring it into every result to ensure absolute accuracy? Or do you visualise a large, rugby-playing undergraduate from Manchester University, paid minimum wage to sit next to, and guard, a large, locked trunk marked “Teachers’ opinions - to be disregarded, as usual”.

Neither image is, of course, true, but we are still left with that puzzling word: “estimate”. In the real world, we also know, don’t we, that estimates are competitive. We might tender for a job of some sort, only to find that someone else’s “estimate” is preferred to ours. Sometimes because it represents equally good work for less money, but often because it is just cheaper.

The truth, as I say, is probably that the estimates are all on some computer programme to be factored in as appropriate. But what weight do you think they will have? Don’t you still find yourself looking at results in August and wondering how certain students have done so much worse or better than you expected? After all, you have known them for five or more years; the estimates have only been on a database for a few weeks.

At such times, aren’t you tempted to scratch your head and utter the immortal words: “Blimey, what bunch of cowboys did you get to do this assessment? You should have got some better estimates.”

Colin Padgett is head of department in an Essex comprehensive