And we've all known the head who sees the classroom as a place of safety, a reversion to cosier former days - a bit like reading your grandson's Beano.
Some heads teach because they want to lead from the front. But such is the pace of change in today's classrooms that a head who tries this is more likely to be trailing behind, trying to interpret the lesson programme that the year-group staff spent their half-term holiday creating. As one year-leader once told me: "I felt like patting him on the shoulder and saying, 'Look, buzz off to Rotary and leave it to us.'"
But perhaps the commonest reason is guilt. There's a thread on the management section of the TES online staffroom forum on which several heads beat themselves up over the issue. One describes fellow senior managers as "very stony-faced - they feel I'm trying to get out of being in the classroom". Another says: "I really punished myself about not teaching when I took up this headship."
It's one thing to feel bad about missing a pre-arranged classroom commitment, but quite another to feel guilty about not being on the timetable in the first place. Modern headship is hugely demanding, and we have to lose some of the old assumptions about how the job's best done.
Maybe there are valid reasons for the head doing some teaching - it's for individuals to decide - but guilt isn't one of them. One head on the forum simply says that if heads are going to teach, it should be on their own terms.
Gerald Haigh's new blog is at: www.tes.co.ukblogmedia