The five days children get off school each year while teachers have extra training should be scrapped as they are failing to raise standards, an expert believes.

The billions of pounds invested in what are often called Inset days for teachers have largely been wasted, according to Professor Dylan Wiliam, deputy director of the University of London's Institute of Education.

Professor Wiliam will call for a revolution in the way teachers approach each lesson at a meeting of leading private school headteachers tomorrow.

He will tell the Headmasters' and Headmistresses' Conference in Bournemouth that teachers need the skill to make "minute-to-minute" changes in teaching technique so that children are constantly updated on what they need to do to improve.

In a report to be published in a US education journal, which the Evening Standard has seen, Professor Wiliam said: "I want to show that, measured in terms of impact on student achievement, the single most important thing to change in teachers' practice is the minute-to-minute and day-by-day use of assessment to adjust instruction."

He said the change, although relatively cheap, would not be easy to make and would have to be gradual.

"Asking a teacher to change what they do is rather like asking a golfer to change their swing - in the middle of a tournament," he said.

For example, research showed that when pupils were asked a question in class, they were most likely to learn from it and be able to give a relevant answer if the teacher left a three second gap for the meaning to sink in. But in practice, even though teachers knew this, most waited for less than one second before asking someone else.

Professor Wiliam said that a teacher with 20 years' experience will have asked 500,000 questions in their career. "When you've done this half-a-million times one way, doing it another way is very difficult," he added.

Teachers, like everybody else, learned most about what the job entailed when they were schoolchildren themselves.

"In the same way that most of us learn what we know about parenting through being parented, teachers internalise the 'scripts' of school as students," Professor Wiliam said.

He recommended that teachers in each school should form Weight Watchers-style clubs, where they set goals for each other in changing their practice to improve pupils' learning - and then report to each other on the progress they had made.

This would take as little as four hours each month and save the country billions of pounds while boosting pupils' grasp of subjects by between two and eight months' worth of progress each year, he said.

Professor Wiliam, who recently came back from a period working in America, has so far conducted only small-scale trials of his ideas and stressed they needed further research. But an experiment involving teachers in the 10 worst-performing schools in Cincinnati produced "statistically significant" improvements, he said.

Inset days - also know as "Baker Days" after Kenneth (now Lord) Baker, the Conservative education secretary who introduced them - were intended to update teachers about the latest techniques and convey government orders for teaching the national curriculum. They are a headache for working parents who have to organise extra child care and many would welcome them being scrapped.

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