“Teachers - you’d expect them to be deeply suspicious of private industry - and all us lot, of course,” says Shifra Cook, of the media buying agency Red Media, which specialises in up-market product sectors. “Us lot” means ad-land: advertisers, their creative agencies - who dream up the campaigns - and their media agencies, who decide where to put them. She also suspects teachers “wouldn’t go to very expensive places for dinner”.

Politics and money are the key focus when advertisers think about teachers.

They are often seen as anomalous members of the middle classes, public-sector workers assumed to have traditional SeventiesEighties pre-New Labour “leftie” attitudes. They are seen as hostile to wealth creation and market populism and likely to inveigh against advertising and branding among their pupils.

And, in a bubble world where pound;100,000 a year for a family man in his thirties is seen as just getting by, teachers’ perceived pay-scales of roughly pound;20,000-pound;40,000 in the ranks provoke indrawn breath.

It’s almost as bad as the Church.

If you’re peddling aspiration and mass-market luxury, this is dispiriting.

Modestly paid secretaries, advertisers know, will pay hundreds of pounds for a crucial handbag from, say, Mulberry or Louis Vuitton, because they have the proper aspirational attitudes (and read Heat, OK! and Grazia). But teachers, so they suspect, haven’t got faith in luxury and are assumed to be “maxed out” on mortgages and improving travel and media.

Along with the politics and the money go those media choices: they’ll read the Guardian and the Observer; they’ll buy Penguin and Virago paperbacks; they’ll be active on the internet; they’ll like art-house cinema; they’ll watch BBC2 and BBC4 - Channel 4 is divisive - and the artier strands of multi-channel TV (though probably on Freeview - cheaper and has a higher moral tone).

The grandees of ad-land, people who start on pound;400,000 a year before the bonuses and options, are more likely to tell you how shocked they’ve been by, say, a few minutes of the NUT conference on Newsnight. The hostility to government and industry, the apparent barminess, all leave a lasting impression on people whose social model - however they actually vote - is overwhelmingly a post-Thatcher centrist one.

And they’ll tell you about the dress code, too. Kit is crucial in ad-land, and teachers are seen to wear horrible clothes. And beards. While describing the terrific vocational commitment of teachers they have actually known - overwhelmingly in the private sector - advertising people cannot help mentioning the last refuge of the flat Mary Jane shoe, the Julie Covington pixie haircut and any other leftovers of “Camden Town taste”. And then there are those men in their thirties who are seen as dressing dangerously like their delinquent charges - shaven heads and baggy jeans. Whatever their own dress codes - and ad-land creatives wear some very silly things - they want teachers to dress like Mr Chips (tweed jacket, leather patches) or the head of Eton.

Then there’s the question of accent and language. The Tims and Simons who run advertising accounts are still overwhelmingly privately educated and RP-speaking themselves (though they may knock the accent back a bit now).

So when they hear public-sector teachers speaking in Estuarine, or heavily regional accents, or using language wrongly (“less” for “fewer” etc) or adopting social-worker-speak or psychobabble (as in “little Johnny’s only carrying a 10-inch knife because he’s suffering low self-esteem”), they flinch. All this is gross caricature, of course, but stereotypes die hard.

Creatives in advertising agencies, however, are particularly interested in how to present teachers because their minds turn to casting - if you have a teacher character in a narrative commercial or a print advert, who should play him or her; how should they be dressed, just how far off mainstream fashion should it be? British advertising is disproportionately built on humour, so the construction of funny teacher types is an obvious angle.

That’s where the beard thing comes in (in Little Britain, universally watched by ad-land creatives, when Vicky Pollard gets gentle career advice, David Walliams’ teacher has a Seventies beard and bad clothes).

Advertising and marketing people feel uncomfortable about teachers. They’re ambivalent about the vocational zeal and the modest pay-scales. They’re bemused that people with tolerable degrees have opted for such an unglamorous career and often think “there but for the grace of God” about teachers they know. But, above all, they see teachers as a missed opportunity - more than half a million well-educated people who perversely do not often come into the target frame, except when you’re selling eastern European short breaks or Rohan hiking gear.

Peter York is an author and social commentator. His latest book, Dictators’

Homes, is published by Atlantic Books