California's policy of cutting class sizes in primary schools, introduced three years ago, has been monitored by researchers who report that children taught in smaller classes are making improvements more quickly than those in larger classes.

The US Education Secretary, Richard Riley, is putting $1.2bn into cutting class sizes

However in schools in deprived areas the move towards smaller class sizes has caused staffing problems. For a lower pupil-teacher ratio, schools need to recruit more teachers, which can be difficult for schools in the poorest areas.

This means that schools have to recruit less well-qualified teachers, which the researchers say can threaten the quality of teaching.

Schools serving poorer communities were also the slowest to cut class sizes, say researchers, because of the difficulties in hiring extra teachers.

But cutting class sizes has proved beneficial among pupils of all ethnic and economic backgrounds, according to comparisons in test results for English and mathematics.

California's programme of class-size reduction now means that 92% of children in the first three years of primary school are taught in classes of 20 pupils or fewer. This compares to a previous average of 30 pupils per class.

This will lend support to campaigns for smaller class sizes by the United States Department of Education and the Department for Education in the United Kingdom.