My next lesson is interrupted by a boy from another lab who needs a stool. Ten minutes later another boy arrives looking for a stool. And so it continues. .. Is there a stool shortage that no one has told me about? And why do only my lessons suffer?
Checking on my lab at the end of the day, I discover the gas man has dug up part of the flooring. Never mind - the schoolkeeper can repair it tomorrow.
Tuesday: The schoolkeeper arrives, looks at the damage, tuts a lot and says it can't be done before Wednesday week. Once more I leave to arrange alternative rooms for chemistry classes.
It had to happen, of course: now I don't have enough stools in my adopted laboratory. I let three late arrivals go off to find themselves a stool each. It takes them half-an-hour to track some down. I make a note to see the senior technician about the stool shortage.
Wet afternoons are never much fun, especially when cooped up with the bottom band Year 9 class - but this is particularly unenjoyable. Our flat roof is leaking, the gap pipe channel has filled with water and the chemistry prep room looks like a scene from Singin' in the Rain.
Wednesday: My admin non-contact period is consumed by showing the borough architect our problem. He scribbles on his clipboard and disappears. By the end of the day, the water has seeped away but the gas pipe channel appears to to have suffered as a result of getting wet. And there are now two warped doors that cannot be opened. I request a carpenter.
Thursday: The carpenter, two assistants and what appears to be half the contents of his workshop materialise midway through first period. This proves a powerful lure for a dozen Year 10 students who find it more stimulating than anything I can offer.
Amazingly, none of my lessons is interrupted by pupils wanting stools and I have enough for all my classes. Checking the carpenters' work, I am pleased to find a lick of paint is all that's needed - which the assistant schoolkeeper has promised to do tomorrow.
Friday: The assistant school-keeper turns up and spends my break time checking what needs to be done. I persuade him also to repaint all the noticeboards in the lab but warn him to keep away from the gas taps.
As I leave, the chemistry technician stops me to say that an absent colleague's class, while covered by a non-science teacher, managed to remove four gas taps from the third chemistry lab. The gas man is booked for Monday morning ...
Martin Wesley is acting head of science at St Aloysius's College, Highgate, north London