In my time as a teacher, advisory teacher and teacher trainer, I have seen some stunningly bad teachers. One was so bad that he could only be given a single student to look after, because with two, there would be a riot.
Another spent all her time in the staffroom because her colleagues judged it was better for them to teach her classes during their free periods rather than deal with the problems that would be caused were she to teach them.
The problem is that I have seen teachers who looked pretty incompetent but whose students always seemed to do well in exams. There were others where the classes always worked well, but somehow this never translated into good grades, and still others whose students did OK, but went on to do very well in later years, even without great teachers. The truth is, we know that teachers make a difference, but we don't know what makes the difference in teachers. That is why the focus on incompetent teachers is a red herring.
Don't get me wrong: I would love to get incompetent teachers out of the profession; their impact on student achievement and aspirations can be devastating, and they corrode the morale of other teachers. But not everything can be a priority. There are three reasons why I think we have better things to worry about:
1. It's not a big problem Chris Woodhead famously opined that there were 15,000 incompetent teachers - less than five per cent of those serving. If we replaced each of these with an average teacher, the net impact on national student achievement would be about one-fortieth of a GCSE grade.
2. It's a complex problem Identifying who is and is not competent is hard because we cannot predict how much students learn by watching their teachers. Ofsted and others claim to be able to do this, but at the moment we cannot distinguish reliably between good students being badly taught and lower-achieving students being taught well.
3. It is time-consuming to deal with If you are going to take away a teacher's job, you have to follow employment law, including rights of appeal. This takes time, and management energy, which would have more impact on student achievement if directed elsewhere.
Dealing with incompetent teachers is like rearranging the deckchairs on the Titanic. Schools are better than they have ever been, and our students are smarter than their parents and grandparents, but the demands of modern life and work have increased even more dramatically (the UK economy is currently shedding 400 no-qualification jobs every day).
Rather than worry about competence, ask every teacher whether they need to improve. If they say yes, support them. If they say no, fire them.
Teachers who do not believe that they can improve are much more dangerous than incompetent teachers because, to protect their self-esteem, they end up blaming the students. They are the teachers who say things like: "What can you expect from these kids?"
The great thing about teaching is that it is so complex, so demanding, and so challenging that one lifetime is not enough to master it.
When everything is a priority, nothing is. Let's put our energies where they will do most good for our students.
Dylan Wiliam is emeritus professor of educational assessment at London University's Institute of Education and the face of BBC2's The Classroom Experiment
CHALLENGING THE SYSTEM
'Do it - but do it swiftly and simply'
Sub-standard teachers should be able to be fired within eight weeks, say heads' union NAHT, which has called for an overhaul of competency proceedings.
General secretary Russell Hobby (pictured) says the current system is flawed: it repeats work already covered in basic performance management, it is derailed by staff going on sick leave or launching a grievance against the head, and it lacks a time limit.
"A bold head can usually make it work for them but by and large the current system makes it too hard to address performance," says Mr Hobby. "We should do it - and do it swiftly. I don't think there is a massive group of dire teachers out there, but there are people who should not be teaching and they need to be dealt with as quickly as possible."
Mr Hobby is calling for an eight-week limit to be imposed on the process, which would remain in effect even if a teacher goes on sick leave or launches a grievance claim.
The deadline could be extended once by a further eight weeks only if progress was being made, he adds. At the end of that period, a teacher would be fired if they were still found to be below standard.
"It's a contentious issue, but simplifying the process would make it more effective," he says. "Heads would be open to the process as well as teachers."
However, Mr Hobby defended the use of compromise agreements in more "ambiguous" cases, where the school as well as the teacher should shoulder responsibility for underperformance.
"There can be blame in both directions when relationships break down," he explained. "In those cases, it can be right to help a teacher move on to another school where they can perform better."
Mr Hobby has also called for the retention of the GTC, which is due to be axed. "It is right that a head can say to a teacher that they are not up to scratch in their school, but not that they should never be a teacher," he says. "There's a need for another body to take those decisions."
LAs with lowest number of recorded competency proceedings in the past five years