The classic occasion is an assertiveness course where you're led to believe that saying no is the hardest thing in the world. (Therefore, you needed a very expensive course to be trained how to do it.) In my case it was experts in time and self-management who profferred this advice. Frankly I'm appalled. If saying "no" is good for the individual, it may be disastrous for the organisation, especially a school, which only works because lots of people are prepared to say "yes".
Saying "no" in teaching could be quite easy, because schools manage staff with precious few sanctions - in the end, whatever it is, no one can make you do it. I have known a year head - in a previous school - refuse to set subject homework because pastoral work kept him so busy he had no time for marking. While the head of department spluttered ineffectually, the teacher explained he was too valuable to the school as a year head to be reprimanded for not setting homework. And the headteacher agreed. It would be a brave soul who said "no" at interview to marking homework. But once you've got the job - well, you can see what I mean about good for the individual, bad for the organisation.
But will you hold a tape on sports day? Or accompany the languages department trip to Calais? Or do the make-up for the school play? "No" to these is a doddle - there's no money, no prestige and you won't be fired, so why bother? We could quote professional development, but it's hard to see applying Panstick in the wings or retrieving youngsters from the beer section of a hypermarket in those terms. We could swear you'd enjoy it, but even we might have our doubts about the truth of that.
No, the real reason for asking you is because things need to be done. Because without help, such things don't get done. Because schools are better places for those things being done. And because pupils' lives, and ours by extension, are enriched by the doing of them.
But these factors are in the domain of the organisation, rather than the individual. Hence my anxieties. We all know "yes" people who have worked themselves into the ground, or, at the very least, the divorce court. And no job is worth your health or your marriage. But I'd rather be a "yes" person, than one who is programmed to say "no". "Can do" is better than "no way" and you never regret the things you do, only the things you don't.
So next time you're listening to a management guru telling you this magic two-letter word is the best way to improve your life at a stroke, have the courage of your convictions, and just say - "no".
Hilary Moriarty is deputy headmistress of an independent school