It’s not an idea we like, but ask any mathematician and you’ll find it stands to reason; most of us are, well, pretty average. Sad really, because what seems to be demanded from teachers now is something called “excellence”. Recent developments in education seem to depend on an abundance of this commodity, last month’s Big Idea - that class size is not important - being a case in point.

Forget excellence for a moment. Leave on one side all those heroes who work 84 hour weeks, all those stars featured in The TES week by week to inspire us. For this (perhaps) 5 per cent of the profession, Mr Woodhead, the chief inspector, may be right. Such amazing performers probably can teach classes of 35, 40, 50 I their planning is so effective, their commitment so intense and - most of all - their charisma so powerful that they rush eagerly to prove what our masters so desperately hope: that if the teaching is good enough, they won’t have to pay for smaller classes.

Disregard also the other exceptions (15,000, we’re told), those at the bottom of our profession’s talent pool who probably cannot effectively teach whatever the size of the class. Say we find a kind way to part with them. That leaves the rest of us, which is most of us.

Alternately told that we are wonderful and useless, we’re actually sort of middling, a motley but serviceable army: regiments of stout matriarchs whose eyes go blurry if they try to mark in the evenings; crabby but kind technology teachers five years from early retirement; dizzy young NQTs full of ideas, who will one day be really good; harrassed thirty-something heads of departments who can’t get on top of their paperwork; lots of decent, skilled and - by definition - averageish teachers.

He can’t sack us all, surely? And yet week by week, month by month, we can sense our jobs becoming more difficult. Last month the big difficulty for us (the bulk of the workforce) was to be told that class size is not an important issue. That simply does not make sense to the average teacher.

Classes in my mixed comprehensive are, for the time being, below 30. We could increase this by five children a class, saving I oh, stacks, I suppose. Class sizes would still be mostly below 35. But all we middling bods would find the job harder. Some might stretch another painful bit; many would just not do as well. Some good things would just not happen.

Of course, such issues are not clear-cut. Teachers rarely calculate, for example, to spend slightly less time marking each book, or to neglect some books altogether; we don’t plan how much time to spend on the extra disruptive pupil.

Sometimes a watershed number is passed unnoticed, beyond which a good lesson does not quite work. Textbooks will no longer quite go round. One or two more children will not get their question in. Bigger classes make more noise, write on more paper, wear out more carpet, need the nit-nurse more often ... are harder to teach on average.

The teaching that goes on in bigger classes is likely to be less good, all other things being equal. Isn’t that blindingly obvious? Other variables may be plentiful, but how can this one be controversial? To us average souls, it’s baffling.

Mike Hawthorne is head of English at a school in Shropshire