Professor Maurice Galton and his team at Leicester University have challenged previous research, notably that carried out by the Office for Standards in Education, that said there is no direct link between class size and pupil achievement. Their report says this is because teachers are not trained to work with small classes and when faced with fewer pupils they do not radically rethink their strategy to increase the number of interactions with pupils, which is known to improve performance.

As part of their research, they chose seven teachers in the maintained primary sector and teamed them with teachers in the private sector. The teachers - all identified as “expert” or “challenging” by their heads - swapped classes. Two teachers from the private sector declined to teach the larger classes. Four of the six maintained teachers were also watched teaching half their own class, with supply cover for the rest. OClassroom observers then noted the number of “sustained interactions” between teachers and pupils in 28 lessons.

They found that teachers spent more time whole-class teaching, regardless of the size of class. In smaller classes teachers asked the greatest number of challenging questions, provided more factual information and posed more problems. In larger classes teachers spent more time having to control the class.

The teachers said the extra time allowed them to get to know their pupils better. “I just get glimpses at the moment. I have an overall picture but you need to fill in a lot of gaps. You presume a lot,” said one.

Despite the advantages of smaller classes, the type of teaching observed was unlikely to produce dramatic differences in attainment. According to the report, the teachers did not use their extra time to ask more open and challenging questions and slow down the interaction with their students.

Professor Galton believes his evidence argues for greater flexibility in classroom organisation. There need not be a maximum number of pupils, rather children should be given the opportunity to be taught in smaller groups. This could be achieved, given the right staffing levels, by half of them doing sport while the rest do mathematics, for example.

The report takes for granted that larger classes are more stressful. “Every single child added to a class creates additional demands on teachers, leading to less contact time and greater stress levels.”

The report challenges the Office for Standards in Education’s contention that it is only at key stage 1 that smaller classes may be desirable. Professor Galton said: “There may be other reasons why small classes at key stage 1 are desirable; for example socialising children into the school culture and into correct ways for working etc. But in terms of desired cognitive outcomes, the case for small classes at key stage 2 also remains strong.”

Professor Galton said the report showed the need for more research. “A more long-term research project which would split classes in half and evaluate different teaching strategies should decide the class-size debate,” he said.

John Bangs, NUT assistant secretary, said: “It is an exciting report and a step in the right direction. The link between smaller class sizes and pupil achievement is established and the improvement can be increased with different teaching methods.”

Class Size, Teaching and Pupil Achievement, research commissioned by the National Union of Teachers by Professor Maurice Galton, Dr Linda Hargreaves, Dr Anthony Pell and Dr Mark Lofthouse