But then it set me thinking about the whole business of celebrity visits to schools. Kids do furnish a room, we know that, and are a peerless backdrop for those who wish to seem nice; but what is the etiquette on visits, what are the rules, are they laid down anywhere? Do you do modules about it at teacher training college? Do parents have to give written permission, for example, before their children are used as decorative backgrounds in political photo-opportunities (remember the PM, again, in front of a stained-glass window and an angel choir at St Saviour and St Olave's). And just how disreputable does a celebrity need to become before a cash-strapped, back-to-the-wall city comprehensive will turn down an opportunity to have the said celeb launch a new book or record with an artful photo-op with Year 7, or a well-publicised chat in the sixth-form centre?
And indeed, when it comes to sixth-form debates and talks, do you lay down any rules of engagement to the stroppier, more politically or socially aware pupils? If Lord Falconer is turning up to drone on about careers in the law or whatever, how do you make sure - or should you? - that someone isn't going to start asking lots and lots of pointed questions about how much money the Dome lost?
If your local Lord Lieutenant or some passing minor royal graciously agrees to present the prizes, are you going to censor your Republican tendency on pain of detention, or do you honour the ideals of the teaching profession and welcome the free exchange of ideas and beliefs at all times, even if it causes the Lord Lieutenant to turn bright purple and choke on his cucumber sandwiches?
We poor parents at home know little of all this. The younger child comes home saying vaguely, "We had a visitor today. About Parliament. Whass for tea?" And only then do you realise that your hated (or, indeed, revered) local MP has been out winning hearts and minds for the next election but two. The older one may be even vaguer. "There was this cool drug guy", which could mean either a pop star in rehab or a particularly fashionably dressed police sergeant with some anti-cannabis leaflets. Royal visits, I suppose, are better signalled, and I also suppose that most teachers, not wishing to have an apoplectic chairman of governors come down on them like a thunderbolt, will instruct pupils to be very, very respectful and speak when spoken to. But suppose mummy and daddy are in the GM food industry, or cutting-edge architects who disapprove violently of fox hunting?
What is to prevent a bright child responding to a hearty "What a nice drawing!" from the Prince of Wales with a pointed diatribe on where he is going wrong? We do, after all, encourage debate, here at St Swine's ...
Oh well, very good, headmaster. Very fortunately the sixth-form Republican society and the Leon Trotsky group have a large overlap in their membership with the film club, so we have organised a special trip for them to see Belle de Jour on Wednesday afternoon during the Duke of Edinburgh's walkabout.
Oh, all right, it doesn't usually happen. Children are strangely susceptible to the idea of "Very Important People" coming to visit them: by and large they sit up straight and mind their manners.
Years ago travelling under escort with a producer in China I was taken aback to be met by a class of 50 chorusing "Welcome, foreign uncles and aunties!" and breaking into a song entitled "I grow up in a socialist garden". It made a change, you could tell, from the everyday routine. But at least they didn't kiss us.
I feel that somewhere in Iraq there is a primary head who is even now wondering how he or she can tactfully phrase a warning to the class that if President Bush drops by, smiles are perfectly in order but it might be a good idea not to kiss him. At least, not until they find the weapons of mass destruction.