Many former head- teachers, especially those who've retired early, support their pensions with supply teaching. It is - and I can testify to this - a salutary experience.
I myself was water-bombed from a high classroom window on my first day as a stand-in music teacher at a comprehensive school. Then, during a stint at a school for children with severe learning difficulties, I found myself trying to extricate a teenager from the large barrel he had climbed into.
(The saving grace there was that both he and I were helpless with laughter.) Now a good friend, another ex-head, has had a similar baptism.
Reporting to me on the phone, he said, "I don't think I ever was a teacher.
I think it was all a dream. The head asked me whether they listened. I said, `Listen? How could they listen when they were shouting?'"
He was going back the next day, though, determined not to be beaten.
The first reaction is that this is all a reminder of the fragility of the head's authority. Stripped of position, you're nobody - you have the status of Saddam Hussein in court, or Cherie Blair with bed hair, fetching in the milk in a dressing gown.
There's some potential for leadership development here. Suppose my friend had still been in post, returning to his headship after his two days of supply work in a school where none of the children and few of the staff knew him? Would he not have brought back deeper understanding of a teacher's task?
Maybe all headteachers should have to do the occasional session incognito.
It wouldn't be difficult to organise - just a trip of a few miles to another school for a day in the classroom. Most heads profess to enjoy teaching after all, and take pride in doing it well.
"I love being in the classroom," they say. "I can forget all the stress.
And in any case I'm the leading teacher and I have to show that I can do it."
"Yes matey," you think. "That's all very well here, where the boys and girls know who you are. But could you do it across the city at St Grumpy's, where you're just another dishevelled individual with a cheap car and a sketchy grasp of the school's routines?"
"Back to basics then?" I said to my friend.
"Oh, it's further back than that," he said. "It's teaching practice all over again."