There are some people who simply shouldn't be teachers. Actually, there are quite a lot of them out there. The tragedy is that some of them don't know it.
This was brought home to me recently when I sat in on an interview for a social sciences lecturing post. One candidate - let us call her Yvonne - impressed by her complete and utter unsuitability for the job.
On paper she looked fine: right degree; spent a period working in industry; PGCE in adult and further education. And her letter was full of all the worthy stuff you would expect. In the flesh, though, Yvonne was different. In the flesh, Yvonne was a disaster.
For a start, she had zero presence. Now this is a bit of a tricky one because "presence" never appears in the selection criteria or guidance notes for interviews. But without some element of it, you are a goner in the classroom.
It was also clear from the outset that she had done almost no preparation. And if she can't be bothered to prepare for the interview, how much preparation is she likely to put into her classes once she has the job?
One question that will always come up at interviews is about equal opportunities. But Yvonne reacted as if it was the first time the concept had entered her brain.
Well, she said, I don't have anything against anybody. While this may indicate a commendable lack of prejudice, it doesn't inspire you that this person has ever pondered for more than five seconds what "equal opportunities" might mean in the classroom.
Yvonne was so far off the pace that you felt sorry for her. She had apparently applied for lots of jobs, had a number of interviews, but always ended up among the rejects.
This wasn't the last I heard of her. A few months later, I was talking to a colleague at another college when her name cropped up. She was working in his department, he said, courtesy of a staffing agency, and was doing a predictably disastrous job.
Now I wouldn't want to imply that all agency lecturers - or other workers in colleges that are supplied by agencies - are like Yvonne. Perfectly good teachers have reasons for wanting, or needing, the flexibility and lack of commitment that agency work brings. But the suspicion lingers that there are teachers doing agency work because no one else will have them.
That point about commitment shouldn't be overlooked either. It's a key factor in good teaching and one you are less likely to get from an agency teacher who is paid by the hour and - not unreasonably - is likely to work by the hour too.
Many colleges know this, just as much as teachers do, and use agency staff judiciously as a fall-back or stopgap when it can't be avoided. Others, however, are much more gung-ho, preferring the idea of a malleable and utterly fireable workforce to one with stability, permanence and rights.
Colleges are currently in the firing line of the University and College Union and Unison, which have been investigating how much colleges have been spending on agency staff. In particular, they have researched the spending of nearly 80 colleges that had, at the time of research, refused to honour fully a deal designed to give low-paid staff a rise of Pounds 550 a year.
It makes interesting reading. Agency bills ran to more than Pounds 1 million at some colleges. On top of that is the amount they have been spending on consultants - in some cases also more than Pounds 1m in 2007-08, the unions say.
No doubt the colleges concerned can explain how it is all necessary. But to the cleaners, many on less than Pounds 6 an hour, the word "disgrace" might naturally come to mind.