A recent episode of The Bill centred on a veteran teacher who was finding it difficult to adapt to the advent of design technology (“I’m just an old woodwork teacher”). The headteacher, was seeking ways of getting the skids under him and the Sun Hill heroes became involved when his alleged assault on a pupil opened a tempting window of opportunity.

The episode revealed just how difficult it is to assess at a glance whether or not a teacher is up to the job. The craft teacher, once respected, was fighting to keep up with curriculum change. Colleagues knew his qualities but saw and feared what the head was trying to do. The head, frustrated by her inability to quantify what she saw as the teacher’s failings, was latching on to a different way of solving the problem.

It is a scenario that will ring bells in staffrooms. No-one doubts that one way to improve our schools would be to sack poor teachers and replace them with good ones. The notion has the double allure of being cheap and red-blooded. The trouble is, it is difficult to make accurate judgments just by watching a teacher in action. To deal formally with a failing teacher takes time, money and emotional energy.

Headteacher Mark Jones, now retired, knows all about these realities. He talks calmly about his experiences of managing incompetent teachers - the ones that are said to number 15,000 nationally. But when I ask if the cases affected him - “Did you worry at home, and have sleepless nights?” - he looks me straight in the eye. “Oh yes,” he says. “It’s bound to be like that.”

Typically, these feelings are more to do with the effect on children than with the process itself.

Shirley Robinson is another head who makes a point of tackling incompetence. She admits: “I get frustrated if someone messes up the process and we all get back to square one.” In 1994 Mrs Robinson started the “capability procedure” against a teacher who, two years on, has countered with a formal complaint about her handling of an earlier stage. “So now,” she said, “I have to appear before the governors to defend myself.”

How does classroom competence - officially called “capability” - show itself? To what extent is it to do with order and discipline? Preparation and marking? Following school policies? Turning in good exam and test results? Being polite to senior management?

It includes all of these, and a lot more, hence the challenge for heads and governors in gathering evidence that will stand up to the rigours of employment legislation.

Once a teacher’s competence is in question, for example, they must be observed at work in the classroom. As David Sedgwick, honorary secretary of the National Association of SchoolmastersUnion of Women Teachers in Coventry, points out: “We all know that the most experienced teacher can spend a long time preparing a lesson only to find that it just does not work. With another group, on another occasion, it might go beautifully, but today it does not. So to anyone watching, you look a failure.”

This means, says Mark Jones, that observation has to be thorough and precise. “The head or head of department has to set aside time - the most precious of commodities - in order to do it properly.”

Mark Jones made detailed notes, including a record of the exact time of each incident. He also described what individual pupils were doing. Then he gave a copy to the teacher and discussed it with them. “The creation of this written observation file is crucial,” he says.

Surely, though, incompetent teaching is made highly visible by restless classes; scrappy work; noise; shouting; indiscriminate use of demerits and detentions? At one level it is. But there are more insidious problems.

In a long, drawn-out case at Shirley Robinson’s school, there were no classroom riots and on the surface, the children were working hard. “The teacher seemed to be keeping up with marking,” she says. The difficulties lay deeper. “The teacher was not delivering what the children needed - not doing what school policy said.”

The first instinct of most heads faced with an inadequate teacher is to reach for the training manual. Mark Jones felt that this desire to approach the problem developmentally rather than judgmentally is inherent in teaching. “Schools are humane establishments, and the basis of our approach to pupils is to use understanding and compassion - to be tolerant and to encourage. It is not desirable that we use any other approach to our colleagues.”

He speaks with feeling, though, about “crossing the tolerance threshold” - reaching the point at which grumbling about a teacher and dealing with him or her informally is not enough.

“At that moment, you decide to put your foot down and start the procedure, because you’ve had one incident too many.”

When this happens, however, it is a mistake to think that the problem has been dealt with. “There is the sudden realisation that you are not at the end of anything but at the beginning, because the formal procedure takes a long, long time,” Mark Jones says.

First, a discussion is usually held with the teacher about their shortcomings, with advice on training and improvement, and notification that performance will be monitored. This might be enough to make a change. But warnings can follow, as well as meetings with the head and governing body committees. It can end in dismissal.

Mark Jones believes that although this process might be long, to invoke it often gives a teacher a jolt. “The prospect of the inexorable progress of events, with no way out, had a tremendous psychological effect.”

But, he points out that the management structure of a school can set up complications. It can happen, for example, that senior managers become more convinced than the head of department that something must be done. “The head of department may then take the criticism personally as a failing on his or her own part, and even start defending the teacher against the management.”

Where colleagues need to be pushed into action, says Mark Jones, complaints from parents “can provide a good lever”.

Shirley Robinson, too, takes parental comments seriously, and also listens to pupils. “Particularly as they get older, they have the right to express their concerns in a proper way, to their tutor or perhaps to a faculty head.”

Once the formalities are invoked, time must be given at each stage for improvement. Targets are set and monitored, and warnings can be repeated. At any stage, the teacher might improve or leave - dismissal is unusual. “We rarely needed to get to the end,” says Mark Jones. “An older teacher might find a way out through early retirement. A younger one may be pushed into resolving his or her lifestyle problems.”

David Sedgwick, who supports several teachers a year through capability procedures, remembers many who have left the profession but none who have been sacked. The NASUWT, it seems, once convinced that the teacher’s departure is inevitable, will concentrate on achieving a negotiated way out.

In the background of these cases, according to Shirley Robinson, there is often ill-health. “The teacher will say, ‘Look, I’m sorry I’ve not told you, but I’m not well.’” Once this is in the open, it can be dealt with, possibly by ill-health retirement.

So where, in this complex picture, does the Office for Standards in Education’s marking scale fit in, grading teachers on a scale of seven from good to bad. The teachers I saw in The Bill, after all, believed that a bad OFSTED mark would put them in danger of dismissal. But an OFSTED spokesperson is emphatic that a low mark does not mean dismissal, “any more than we expect a good teacher to be automatically awarded a bonus”.

The most that OFSTED’s grades can do, it seems, “is provide additional management information. It may confirm a head’s own impressions, or indeed the head may decide to ignore it on the grounds that he or she knows better. ”

Shirley Robinson, in common with many other heads, doubts the system’s value as additional evidence. “It’s a judgment made at a particular moment. Three lessons have to be observed, two of which have to be bad - just two. How could I call that up as evidence? I cannot see how it assists the head,” she says.

She believes that Ofsted, by becoming involved in judging individuals is compromising its position. “Ofsted should not have anything to do with competency at the individual level.”

None of the headteachers criticised the role of the teacher unions. They assume that union representatives, usually experienced and able teachers themselves, will not strive to keep a useless teacher at work but may insist meticulously on fair dealing. As David Sedgwick put it: “It does not help the teaching profession if there are people in it who are not capable - it does our case no good.”

However, he believes that capability cases are increasing, and that this is partly because heads who aim to get rid of someone who is too expensive, too old, or whose personal style does not fit the ethos of the school, abuse the procedure.

He suggested that the procedure can be used against a teacher, for example, by setting carefully chosen pernickety criteria, which might tax any average teacher.

He recalls a teacher who was said to have left some books unmarked. “I said that there were probably some unmarked books in my classroom, too. They often set up the ideal as the target, and let’s be honest none of us is ideal. ”

There are, he finds, great contrasts in the way heads approach capability. “Some deal with the issues in a fair, reasonable and generous way, trying to meet the needs of the individual so as to raise their capability. There are others, though, who use it in ways which are immoral, and for their own ends - using it in a deliberate way to weaken individuals who need support.”

What often gives these heads away, David Sedgwick finds, is their use of a formula that he despises. “The head will say, ‘The staff have had enough’. It’s a terrible indictment of the head when they say this. What they mean is that they have had enough, but lack the courage or the backbone to say so. I’ve had this used to me with monotonous regularity and I think it’s appalling.”

Given all of these complexities, what chance is there of achieving the aim of many outside the profession - that of getting rid of poor teachers with a minimum of fuss? In the immediate future, probably, not much. As Shirley Robinson puts it: “When I see damage being done to children I feel guilty at not being able to deal with it quickly, but I fear it will take much greater legal minds than mine to come up with a way of doing it.”

The names of Mark Jones and Shirley Robinson have been changed.