I haven’t been doing much supply work lately - for several reasons. Writing a Hollywood blockbuster is one; taking the dog for long walks is another. But I still do the odd half-day to keep in touch, and the two sessions I did a couple of weeks ago illustrate perfectly the Alice in Wonderland nature of the job you all do out there.

Even supply work is dog’s work now. There was a time when you took in your own work, plenty of story-writing and problem-solving maths sheets. If you played the guitar, you’d end the day with a singsong. If not, you’d at least read them a story.

You found a clear desk waiting for you when you went in, and a note saying: “Have a nice day and enjoy yourself. Hope they’re good.” So you did your thing and left a tidy pile of completed and marked work for the returning teacher.

How things have changed. I arrived for this week’s first morning to find the teacher in the classroom reading through a great planning file and writing out the day’s work. Literacy first in three groups. I groaned. One group reading together - the group I would be with. Two other groups each doing different comprehension exercises from different books. How do I introduce and go through two comprehension exercises with two groups if I’m sitting with another group reading the set text? There were too few books for the set text. So piles of photocopying lay ahead to sort through while I simultaneously spoke to two groups about two different exercises.

Somehow we started, with me ordering the two comprehension groups not to interrupt while I sat with the reading group. I plodded through a book which no amount of animated reading could bring to life. At the end, I gathered in the comprehension, having left the readers to work alone. Only a few had done the work satisfactorily.

An hour remained to “do” history. Piles of photocopied sheets about ancient Egyptian journeys to the afterlife had been done for me. The hurried approach, the token hour, made me despair. Why doesn’t someone invent a subject and call it historacy? Class reading, set texts, comprehension, ancient Egyptians - here was a golden chance to bring literacy and history together. Here’s something as exciting as the ancient Egyptians. Here’s something as spooky and macabre as journeys to the afterlife, being judged by Osiris, balancing your heart against a feather, and being led by Horus, if you tell the truth about being good, to eternal life. But it was a chance missed.

I despair at the brainwashing now, the pigeonholing, the rigidity, the straitjacket you’re all in. It’s in the book, so we do it this way and say: “Ooh, you can’t do that.”

The next day, another class. There was the work laid out for me, but too few books to go round, so more photocopying. I knew what the topic was for maths so I took in a pile of my own material. The teacher looked surprised. “Oh sorry, you can’t do it that way. That doesn’t come until the end of the book.”

The literacy work was all laid out. I introduced it to the class. “But we’ve done this already,” they cried. “With the other supply teacher.”

Change of plan. Quick. I rub my hands in glee. I switch to a piece of writing for the whole class about the blackened, derelict shed at the bottom of the garden, leaning over alarmingly at 45 degrees, with something shadowy lurking in the corner. I tell them about the tumbledown shed in the garden I had, the hens and rabbits my father-in-law reared in it during the Second World War. About the old tramp who used to sleep in it, the foxes who made dens underneath it, the birds’ nests, the hibernating hedgehogs, the heaps of rubbish and junk that would now fetch a fortune as garden antiques. And the shadowy thing lurking in the corner? A stray dog curling up for the night. They sat spellbound, lost in the imagery and treasures of that 60-year-old shed and the strings of new words.

And then, when I said you go and write about an old shed at the bottom of your garden, they were like puppies let off a lead. The stories are in my bag for me to read tonight. I don’t think I heard a sound in that room for the rest of the morning. I was naughty and cut short the maths hour. I was naughty and added the time to the literacy hour. Please forgive me.

David Thomas is a retired primary head in Leeds.