Some time ago, during the equally torrid affair of the drama teacher who skinny-dipped with teenage boys on a school trip and got so rat-stinking drunk that she had to be put to bed by a colleague, I wrote a column here with a sad, hard moral. It pointed out, regretfully, that of all professions in an increasingly infantilised and petulant world, teaching remains one of the few in which it is necessary to be a grown-up.

However wild a party animal you may be at heart, however full of “needs” and “issues”, however loose and carefree and multicoloured your natural personality, if you’re a teacher you have to can it. You have to dress in respectable clothes, keep order, count heads, moderate your language, and watch your behaviour - as they say in business - 24-seven. On the face of it, the affair of Big Brother contestant Penny - otherwise known as Lisa Ellis - of the Sarah Bonnell community school is just another example of this harsh rule. Her headteacher has glumly said: “I put it in writing to her that she would have to adhere to the code of conduct in her contract... I have to ensure the pupils have excellent role models”. She has also observed that she believes that the teacher may have “decided to go down the road of enhancing her career in other directions”, to wit, showbiz. Ms Ellis herself has indicated as much on screen, making it pretty clear that like most other contestants in this hideous programme she is out for instant celebrity and whatever it can bring her. So good luck to her, and may her slipping towel bring her all that she desires.

But it brings up the eternal, vexed question of the Showbiz Teacher. The trouble with schoolteaching is that it is part scholarship, part showbiz; it always was and always wll be. Given that even in the most selective of schools some 50 per cent of any set is not sincerely interested in your subject, you gotta sell your pitch. You need presence, you need originality, you need communicable energy, you need rhetoric and verbal power - you need a lot of the same things, frankly, that Denise van Outen uses to sell a song in Chicago. Admittedly razzle-dazzle alone doesn’t get people through their physics GCSE, but a bit of fairy-dust doesn’t hurt. The teacher we remember forever is not the dull, strict, mumbling pedant, but the ranter, the performer. Everyone benefits from the apparent nutter who makes you stand on the table and shout Latin words, the history teacher who careers round the room throwing things and then makes you each write a brief account of what he just did, in order to demonstrate the flawed reliability of eyewitnesses. It is, perhaps, generally better if most teachers refrain from actually singing or performing the can-can, but a bit of razzmatazz goes a long way.

Therefore, in any feisty staffroom, there are at least one or two members who have yearnings to take it further: who carry in their teacherly knapsacks not a headmaster’s baton but the tatty tickling-stick of a Ken Dodd, the microphone of a crooner, the sheer black tights of a soubrette.

To hell with Lord Puttnam’s teaching Oscars, they mutter: what about the real thing? And some of these frustrated performers (not only English and drama staff either) are likely to be very good teachers. So what is a head to do, in an era of staff shortage, when one of them announces that he or she wants to go on Big Brother, or take a sabbatical Christmas term for a panto engagement, or host a saucy local radio phone-in slot of an evening, or spend the summer holiday in seaside rep? Thwart them and they’ll build up a steaming head of frustration and eventually make a run for it. Give them their way for a while, and they might just realise the desperate, miserable precariousness of the true showbiz life and return to entertaining an audience which can be put in detention for heckling.

It’s a tough call. I watch, fascinated, to see whether a season in the sun has Penny limping back to the classroom, strangely grateful.