You don't need complaints from parents to know that something's wrong. A teacher is missing lessons week after week, preparation is sketchy, marking is patchy and the children are losing out. At such moments, a headteacher has to act.
Don't put off those tough conversations. Although there may be tears, you must act - kindly, to be sure, but firmly too. But what happens when the culprit is someone close to you? Actually, closer than close: what happens when it's you?
I found myself in that position 10 years ago. When I started out as a headteacher 10 years before that, I decided not to teach: it was an internal promotion and I had to remove myself from the staff body to a visible extent. I told myself that it was the price of leadership (I'm noble like that).
Then an RE teacher fell seriously ill mid-year. What could I do but step in? The next thing I knew, I was back on timetable.
But then came that moment, a decade ago, when I met a hugely experienced fellow headteacher on a station platform. We exchanged pleasantries, then he looked at me sharply and asked: "Are you still pretending to teach?"
I was. By then I'd moved on through Year 7 Latin and German to beginners' Italian in Year 9.
My inquisitor went on: "So I suppose you've had to set work while you miss another lesson? Then there'll be the marking to do when you get back. And then you'll find you've got to miss the next lesson anyway. You're doing it badly, the kids get a raw deal, and you feel lousy about it."
He was right on all counts. Don't you hate it when people do that? So I took myself into a corner for a stern chat: "You miss lessons all the time, you don't prepare properly, you're behind with marking - it's got to stop." Tears and entreaties followed, but I was implacable. I sacked myself from teaching.
After a period of trauma I began to feel better. No guilt, no sense of a job done badly, no pressure to do it better. I didn't get to know the Year 7 students so well; I had to learn again that such disconnection is the price of headship.
Then I moved to a new school. I didn't teach. But I did feel that I should contribute to the sixth-form enrichment programme, so I set aside some time.
Inevitably, the guilt returned. I got busier and it got harder to protect those Tuesday afternoon sessions. In my head I heard the echo of my old colleague's advice, and last spring I nearly heeded it. I was all ready to sack myself for a second time (I'm tough like that) until a parent stopped me in the street and said how much her son loved my sessions. So I carried on.
Last week was a bad week: everyone has them. The parents all seemed to be dissatisfied and my colleagues were fractious. Under pressure for time and sorely tempted to cancel the session, I took my group to a nearby art gallery. They were delightful company and the best bit of the week. Why should I deny myself that?
But the fact is, I'm writing this on the train to London. And missing my session with those sixth-formers. And feeling bad about it.
Bernard Trafford is headmaster of the Royal Grammar School in Newcastle