I wouldn’t mind dividing pupils into sets if it were straightforward. Oh, to teach maths and use numbers to make the decision. A child has an average test score of 86. She should be in a higher set than a child with an average test score of 28, right? Right.

But I don’t teach maths; I teach English. And it just ain’t that easy. Every year I lie awake and sweat as I think of the enormity of the task ahead: putting Year 8s into groups where they will probably stay for their entire school careers.

Our small, rural secondary does not have 32 pupils who are brilliant enough to be labelled “top set” and whose talents clearly distinguish them from their peers in set 2. I can live with the decision to put the truly talented in the top set, but there are just four or five children at most in each year group whose brilliance no one could deny. Then there are a dozen more bright kids whose “safe” level 5s at the end of key stage 2 could not be disputed, which gives me half a class.

How do I fill the rest of that top set? I have a cluster of kids who, the spreadsheet tells me, scraped a level 5 in KS2. Closer scrutiny of the data reveals that several of them came from one large feeder school that has a generous Sats marker. The darlings can’t tell a correctly spelled homophone from a piece of Stilton and certainly can’t organise their ideas into cohesive paragraphs.

So, how to allocate my surplus 15 places in the top set? How do I select from the pupils on the 45 border? Neat handwriting? Good behaviour? Or do I let other factors cloud my judgement? Pushy mother? Aggressive father? Parental tendency to deliver chocolates at the end of term?

And I haven’t even started on the bottom set. Now there’s a can of worms ...

The writer is an acting head of English in the South East of England. To tell us what keeps you awake at night, email david.marley@tes.co.uk.