"The phantom sandwich flusher is back," he says, with all the gravity of a doctor breaking bad news. "End of every day this week. Just like last term."
I take a deep breath and cough as the disinfectant fumes catch at the back of my throat. It's the worst possible news. The return of a mystery that remained unsolved last July.
"Year 5 boys' toilets," continues the caretaker, assuming the role of Lewis to my Inspector Morse. "Cubicle three. Brown bread. Cheese. Possibly cucumber." I warm to my role and my questions flow naturally. "Wholemeal or granary?" The caretaker's mop twitches. It's unfamiliar territory and he can't be sure. He shakes his head and turns, painting a horizontal stripe of dirty mop water across my trousers. "Triangles or rectangles?" I call after him. Without turning, he calls back. "Rectangles."
I put together the few leads I have for my investigation. Why, when there are bins every 20 paces all over the school, does anyone want to flush away uneaten sandwiches? And why do they not stick around to discover that they float?
The next day, I ask my year colleagues to inspect every boy's lunchbox, looking out for cheese on granary. Three suspects emerge and I expect a result by home time. The bell rings and I seek out the caretaker brandishing the names of the suspects written on a piece of paper. I find him in cubicle three, on his knees, wielding a sink plunger like a demented Dalek. He shakes his head. "White. Sliced. Jam. Rectangles," he intones and carries on thrusting his arm around the U-bend. I return to the classroom deflated, but decide to circulate a proforma entitled Boys Sandwich Nutritional Survey.
There are three headings: "Name", "Bread type", "Filling", and "Cut shape".
It's quite surreal, but I'm going to nail the phantom even if I get a week behind with my marking. The results are an eye-opener and a mouth-closer.
There's the standard chocolate spread on white malted. Then there's tuna in a sesame bap. Marmite on bagel sits beside kebab sandwiches on wholemeal. A spaghetti bolognese baguette turns my stomach. Some lunchboxes contain just chocolate bars, cheese dips and processed drinks. The message from the Healthy Schools Initiative isn't getting through.
But there is a granary and cheese. Just one.
At home time, I let my class dismiss themselves and I run to cubicle three, slamming the door open as if entering a saloon. I peer down into the bowl.
It's empty. I stifle a scream and flip down the seat. And there they are.
On the ledge behind. My beauties. Two perfect, rectangular, granary, cheese-filled sandwiches. I emerge clutching exhibit A above my head like the Holy Grail. A Year 5 boy decides against taking a pee and hurries off home.
The next day I confront the suspect with the now curly sandwiches and he confesses, down to the last crust. He's just not hungry at lunchtime, and by putting them down the toilet instead of in the bin, he can't lie about throwing them away. Twisted, but logical.
I'm left with an overwhelming sense of triumph over a tiny but important cog in the machinery of problems of a head of year. So many are relentlessly on-going, open-ended or unresolved. But this was a result.
Colin Dowland teaches in a north London junior school