Such group sessions which have been compared to WeightWatchers meetings would give students the equivalent of eight extra months’ teaching a year, said Professor Dylan Wiliam, deputy director of London University’s Institute of Education.

In-service training days should be banned, he said. Instead, schools should set-up “teacher learning communities” groups of up to ten teachers who meet for two hours a month with the aim of improving their teaching.

The “WeightWatchers” sessions so nicknamed because they would be mutual support groups of people striving for a common goal would give teachers feedback on new techniques they had tried and an incentive to continue trying to improve, Professor Wiliam said. A further two hours a month should be spent observing each other’s lessons.

A trial carried out with 24 teachers in six schools showed that pupils progressed at almost twice the normally expected rate because of their teachers’ exchange of ideas.

Professor Wiliam told the annual conference of the Headmasters’ and Headmistresses’ Conference, a group of leading independent school heads, that the teacher sessions would be a much more cost-effective way of improving attain- ment than smaller class sizes.

Cutting class sizes from 30 to 20 pupils would give children an average of four extra months’ learning each year, but would cost up to pound;20,000 per class.

But Professor Wiliam said the groups resulted in eight months of extra learning per year at a cost of only pound;2,000 per class.`