“Remember you’re out!” I yell. “We’re representing the whole school - the entire state system!”

I insist on perfect manners. They nod. I insist on perfect behaviour. They nod. Then they invade the station like barbarian hordes. They zoom down the up-escalators, and up the down ones. The public looks on in horror. We get to the platform. They josh and push each other to the edge. They flirt with mortality. I’m not paid enough for this.

“Remember you’re out!” I thunder. The train charges in. And so do they.

Passengers wince and stare. My pupils are cartoons of their worst nightmares. A Daily Telegraph reader makes a mental note to go private.

“Are you in charge of these people?” he barks.

“Apparently not,” says a Daily Mail reader.

“Are they yours?” she trills.

“Good lord, no! They’re orphans. Travellers. Poor mites seeking asylum!”

“Really?”

“Not really. We’re the west wing of the Maudsley. It’s our day out.”

I’m rather pleased with this, and my little chums are rather pleased with me. They’re suddenly on my side and become model citizens. Dave Mania offers his seat to a pin-striped elder. Sabrina offers hers to Mr Telegraph. Crumlin removes his hood and is a pantomime of courtesy. I have a halo of smugness. I smile.

A Guardian reader smiles back. Shaka joins in. He smiles beatifically at the whole carriage. He’s probably stoned again - on something called “skunk”. He leans back against the doors. They slam shut emphatically on his copious dreadlocks. He is inside the train but much of his hair isn’t.

He does not notice these things - the skunk has him in thrall.

His chums regard him with great mirth. The passengers regard him with awe.

The train starts. Shaka’s locks do not seem to have interrupted any electrical circuits. His head is fixed at an angle. I hope he won’t die. I hope he won’t get vacuumed down the tunnel.

Three white-knuckle minutes follow. We arrive at the next station. The doors open. Shaka is released. He does not notice. Like Samson, he shakes his locks. They’re covered in soot and dust. They are dreader than ever. I sigh. He’s still alive. I could kill him.

We finally get to the Natural History Museum. Nothing else can go wrong. Or can it?

You should have seen what Shaka did with the dinosaur.