I’m cycling down the Portobello Road. There’s an inflammatory kerfuffle on the corner. Soft bobbies and high-octane youth are discussing drugs.

“Hello, Sir. Tell him, Sir,” yells a hooded youth.

“Good afternoon boys.” I hail the homeboys, who are alumni. Peace is restored. The police, gobsmacked, move on. Who is this figure in the bike clips? Jesus? A local knightTony Soprano?

I pedal off and the boys continue to peddle their wares - bits of rubber dustbin lid - to the more gullible of tourists.

Or I’m walking home from school and a young policeman hails me. He rifles my bag until he chances upon a register. “Sorry, Sir.” I am now a pillar of the community.

It works on some people, but not always the pupils.

“Have you though, Sir - smoked weed?” Of course I haven’t. There can be no debate. I don’t. You don’t. We don’t. It doesn’t happen. Just like homosexuality and teenage boys. It has never been recorded. If I address it, I might be promoting it. That way madness lies. Or an expose in the Daily Mail.

“Anything though, Sir?” Well, there was a skirmish with the Coca-Cola and ash. I was about seven. We had recourse to this hellish brew to escape the pain of the Home Counties. We drank it behind the bike sheds and it made us feel different. Or the times I went down the mean streets of Chalfont St Peter collecting fag ends. This too induced altered states - a green skin and puking. As for the problem with he Tizer, they wouldn’t want to know.

There can still be no debate.

There seem two options. You can conjure cautionary tales of reefer madness, of rotting basements with a needle and a spoon. Of that fatal journey from passive dope-smoking to the terminal agonies of smack hell. Or you can perhaps risk a bit of louche empathy. This involves lots of street argot and fools no one. It might also involve jail - or tabloid hell.

“You have though, haven’t you, Sir?” I’m a bit better off with an outside speaker, though this too can be tricky. You can get storm troopers from the Drug Czar or ex-junkies. Both are a little dramatic for a Monday morning. A happy mix of Will Self and Joyce Grenfell seems more the ticket.

The pupils know much more than me - about uppers and downers and screamers and things that go wizz. So we make a list. We note that some items make you feel a bit dizzy, while others can fell an elephant, make you see crocodiles or become a cabbage.

But most of them just say no - despite the pressures of the national curriculum. All those Cats, Sats and modules would have me back on the Tizer. Perhaps Widdecombe’s contributions to the debate will open dialogue. Until then I must be muter than a shadow cabinet, and the pastoral lesson must go thus: Outside Speaker and I tell the class drugs are bad. Hands up who agrees. Whole class raise hands. Hands up who has never taken a drug. Whole class raise hands. Hands up who is never going to take a drug. Ditto.

“Well done everyone,” says Drug Czar.

“What, not even weed, Sir?” The writer, who wishes to remain anonymous, teaches in London