Reducing school class sizes to boost pupil achievement is a waste of money for all but the youngest children, a leading education expert says.

Cutting class size from 30 to 20 pupils gives children the equivalent of four extra months of learning per year, but costs about £20,000 per class per year, Dylan William, deputy director of the Institute of Education in London, says.

A more effective solution would be a teaching method called formative assessment, which can provide 8 to 12 extra months of development for £2,000 per class per year.

“Using the right teaching methods can be 20 times as cost-effective as reducing class size. Smaller classes do confer a benefit if pupils are unruly, because fewer pupils in a class mean less disruption. But as long as pupils are well behaved, then what you can do with a class of 20 is generally possible with a class of 30,” Professor William will tell the annual Chartered London Teachers Conference today.

Professor William’s findings question the Government’s new drive to introduce personalised learning into all state schools using “assessment for learning”. This is based on regular tracking of the progress of individual children and setting of individual performance goals.

Rather than attempt to track how individual children are performing – a task virtually impossible in a class of 30 – Professor William advocates assessing the performance of the whole class minute by minute, day by day, using formative assessment.

“The personalised agenda is dangerous because it focuses on individualisation. What you can do is to make better assessments of the whole class as you go along,” he said.

“This might involve stopping a few minutes into the lesson and posing a multiple-choice question on what has just been taught. Pupils hold up fingers to indicate their answer. Immediately it tells the teacher if the class is ready to move on. If nobody gets it correct, you teach it again.”

A similar method is the “traffic lights” system in which pupils hold up coloured cards to show whether they have understood. “Alternatively you get the kids to answer a question on an anonymous index card at the end of a lesson. The teacher can see what proportion has understood.

“If you find out during a lesson that 10 pupils have learnt something, but 15 or 20 have not, you can do something about it then and there,” he said.

Studies of formative assessment in the US and the UK have shown it to double the rate of learning. It is being given trials by 60 schools in the UK. The studies suggest that cutting class sizes would be cost-effective only for pupils aged 4 to 7 and even then, they should have 15 pupils or fewer.

Professor William believes that the best teachers adopt a formative assessment approach instinctively. The best way of training the rest would be monthly workshops at a cost of £2,000 per teacher per year.

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