"The Daily Dumper has struck again," shrieks our senior mealtime supervisor, her scarf clutched around her face as if the Ebola virus had been accidently released. "Cubicle five," she adds gravely. "Always cubicle five."
She begs me to take a look, despite my protestations that I believe her, so I'm dragged upwind to the newly refurbished boys' toilets. The letter I was writing to the local council remains on my desk, unfinished.
Cubicle five is like a crime scene. I wrap a hankie around my biro and push the cubicle door open. There it sits, bold and impressive, a metre shy of the bowl and smeared with some expertise across the terracotta tiles. It certainly doesn't blend into the stylish refurbishment. For a nanosecond I consider entering it for this year's Turner Prize, or perhaps the less well-known Turder Prize.
We have kept the boys in twice at playtimes, with lectures peppered with emotive adjectives and disappointment. But after the fourth dump, it is clear that the guilt-trip approach won't flush out the culprit.
I call a meeting and ask for ideas. I erect a flipchart, draw a map of the toilets and start scribbling. Operation Toilet Storm is under way.
"We could stakeout here and here," I say, tapping the map at cubicles four and six.
My deputy shakes his head. "Safeguarding issues."
I concede and scratch out my stick men and circle an area at the end of cubicle five.
"There was a shoeprint just here," I continue, "On top of Tuesday's turd."
"Not enough definition," sighs my site manager, who joins the meeting, peeling off his rubber gloves and flinging them into the bin. "Can't get a positive match."
"How about genetic stool analysis?" suggests my science leader, typing a Google search enthusiastically into her laptop.
"Whole school?" I ask.
She nods her head and hits the return key.
"Difficult logistics," warns my deputy. "Permissions slips, the works."
"Toilet monitors," pipes up my business manager, who has been sifting through hours of CCTV footage. "They could record who goes in and who comes out and keep a log."
At the word "log" everyone winces.
Our child psychotherapist stumbles into the staffroom, struggling with a box of old plastic toys for the art therapy room.
"Maybe I can be of help?" she says, removing a large plastic banana from the box.
We all turn towards her and put down our notebooks and black coffees. She is our very own criminologist, our very own Cracker.
"Look at all the evidence," she says.
"I"d rather not," I hear someone mutter.
"It's a classic profile. Freud would have a field day," she says. "I think we are looking for someone quiet, but very angry. What they're doing is a protest. And it's pretty effective."
The room is silent for a few moments.
"You know that letter I was writing to the council, to protest about the cuts to the children's service?" I announce. "I've got a better idea. Anyone got any All-Bran?"
Colin Dowland is headteacher of a junior school in north London.