Reducing class sizes at key stage 2 and above is a waste of money without good teaching practice, according to new research by a Welsh-born teacher-educator.

Professor Dylan Wiliam, deputy director of London’s Institute of Education, believes formative assessment - teachers understanding and delivering the needs of pupils - is 20 times more cost-effective in raising achievement.

The academic, who has spent three years researching class sizes in the UK and the United States, says teachers who are in tune with their pupils can provide eight extra months of educational development for pound;2,000 per classroom, per year, compared with the pound;20,000 cost of cutting class sizes by 30 per cent, which he says amounts to four extra months of learning.

But Professor Wiliam admits bad behaviour could act as a spoiler in a class of 30 pupils or more where there was good formative assessment. He also told the annual Chartered London Teacher conference last month that classes of 15 pupils were vital for under-sevens.

Figures from the Organisation for Economic Co-operation and Development, published by the Assembly government earlier this month, show the average primary class size in Wales is 24 - two more than the average (22) of the 32 developed countries surveyed.

Wales ranks ninth, behind the USA, Poland, Portugal and Australia, but is still ahead of the UK, whose average is 26.

At present, there is no legal requirement for junior class sizes in Wales to be 30 or under, but the Assembly government has set it as a target. It does, however, make a commitment in its 2007 One Wales document to “deliver radical reductions” in class sizes. This is particularly geared to the play-led foundation phase roll-out with a 1:15 teacherpupil ratio.

Professor Wiliam’s conclusions, praising the virtue of good teaching practice over class size for KS2 and above, will back officials’ claims that class size is not the most important factor in achievement.

He gives examples of formative assessment, including the traffic-light system in which pupils hold up different cards to show whether they have understood a lesson. Red means “no,” amber means “partly”, and green “yes”.

Professor Wiliam and his co-researchers found that groups of eight to 10 teachers who meet once a month for at least two years can be most effective in spreading good practice as part of formative assessment. “Simply telling teachers what to do doesn’t work,” the Bangor-born researcher says.

But Kirsty Williams, Liberal Democrat education spokesperson, believes smaller classes improve teaching quality, a view mirrored by teaching unions. The party wants all classes in Wales to be capped at 25 pupils.

Brian Lightman, president of the Association of School and College Leaders, said improved formative assessment should come with cuts in class sizes.

Peter Hogan, warden of the independent Llandovery College in West Wales, also said good results at the college reflected that there were only about 14 pupils in classes for core subjects. The college had a 93 per cent A*-C grade GCSE pass rate last year, 30 per cent above the Wales average.

Figures released by the Assembly government last November show that 58 classes exceeded the 30-pupil target. There were also some infant classes with more than 30 pupils, making them illegal.

In apparent agreement with Professor Wiliam, an Assembly government spokesperson said this week: “We have made significant progress in reducing primary class sizes in Wales.

“A statutory infant-class limit of 30 pupils was successfully implemented in September 2001 and the Assembly government has pursued a ‘made-in-Wales’ target of reducing junior classes to the same level.”