"Are you in on Thursday morning, Miss?" Jack asks casually.
It sounds an innocent enough question, but when Thursday is Farewell Day, believe me, it couldn't be more ominous.
Farewell Day is one of the highlights of the school calendar for Year 13. It's the day when leavers traditionally create havoc for two periods before saying goodbye and being escorted from the premises on exam leave, returning only to sit their A-levels.
Having spent their school career in keen anticipation of this moment, they are eager to excel in the task - nowhere more so than in the library, which has for some reason become a focal point for Farewell.
Knowing the chaos that may ensue, staff could be tempted to phone in sick. But the chief librarian operates a three-line whip for the occasion, and no amount of coughing or retching over the phone will convince him.
Planning for the big day starts weeks beforehand, as I can testify from the snatches of conversation I've overheard during my perambulations. They go something like this:
"But how can we make sure Mr Simpkins is still in there when we light the match?"
"We'll need three of us, though, 'cos someone's got to hide the slugs, innit?"
"I can bring the ladder, but I'm not going up there, it's too effing high ..."
"Yeah, but food poisoning can be really serious, y'know ..."
I remember one year, we were forced to barricade ourselves in the library while stink bombs appeared under doors and a personal alarm was lobbed, shrieking, through an open window, its shrill call unstoppable as I wrestled to break it open and remove the battery. As I emerged tentatively an hour later, it was to the flash of cameras and pleas for my autograph from the by-now-tearful 18-year-olds.
"Will you miss us?" they always ask.
"Of course I will," I say truthfully. For it would be impossible not to miss their laughter, their boisterous retorts, the frowns of concentration as they hunch over their books.
And I know how scared many of them are to be leaving the place that has helped them grow from small, hesitant newcomers into tall, surefooted (but still slightly hesitant) sixth formers.
Next stop university, although for many there is the prospect of a gap year first.
"What do you think I should do in my gap year, Miss? Should I go travelling or just work and save up so I can pay the fees more quickly?"
I try to be diplomatic and resist the urge to tell them to sod the fees and grab life with both hands - this could be their best chance of freedom.
"Maybe you could do both," I say. "Maybe you could work for a bit and then travel, but I shouldn't worry about money just yet."
Somehow, though, I think my advice falls on deaf ears. Quite apart from the fees, those that are planning to travel are wondering how to fund it. But some are growing inventive.
I once arrived on the last morning of the summer term to find a posse of leavers slumped on the pavement outside school, busking tunelessly. Next to them was a cap labelled: "Gap Year Funds - thank you."
Claudia Court works in the library of a London secondary school.