Parents have long held it as a sign that their child will get first-class teaching and all the attention they need to succeed in lessons. But smaller class sizes have only a "small" impact on pupils' attainment and behaviour, according to a study by the Department for Education.
Even then, the benefits are positive only in the early years of school and diminish "after a few years", the publication says.
This means that using budgets to create smaller classes is not the best use of funding.
"Overall, the available evidence suggests that class size-reduction policies are not the best option in terms of value for money for raising pupil attainment, compared to others such as increasing teacher effectiveness," it adds.
The Class Size and Education in England evidence report was written by the economics, evaluation and appraisal team at the Department for Education - part of its education standards analysis and research division.
The report says that "there is not enough evidence" to determine whether the relationship between class size and attainment varies for different class sizes.
"For example, a greater understanding is needed on whether or not a fall in class size from 25 to 20 has the same effect as a fall in class size from 40 to 35 or whether families might compensate for larger classes," the report states.
Previous research in English schools, cited in the study, found that, in smaller classes, individual pupils were the focus of a teacher's attention for more time, there was more active interaction between pupils and teachers, and there was more pupil engagement.
Studies have shown that, in larger classes, more time is spent on pupils interacting with each other, more time is spent on teachers teaching the substantive content of the subject knowledge, and more time is spent on non-teaching tasks such as taking registers.
Smaller classes have been found to lead to a slight increase in the number of years a student spends in post-compulsory education. A study from Denmark estimated that a reduction in class size by 5 per cent during the whole of compulsory schooling (from an average class size of 18) provides a rise in post-compulsory education of approximately eight days.
The DfE study also shows how class sizes have changed over time, the impact of the increase in birth rate on pupil numbers and how this could affect the teacher requirement and class sizes, and the impact of class size on educational outcomes.
There has been a legal requirement on schools and local authorities to limit the size of infant classes taught by one teacher to 30 pupils since September 2001.
The percentage of pupils in classes of more than 30 pupils decreased significantly between 1998 and 2002, and has remained very low since 2002. The majority are in "lawful" classes, which have been permitted to exceed the maximum.
The proportion of pupils in unlawfully large classes has wavered between 0.3 per cent and 0.7 per cent every year since records began in 2006. In 2002, 26 per cent of pupils were in classes of 30, with just 1 per cent being taught in larger classes.
There will be increasing demands for primary and secondary school places over the next few years because the number of children born in England has increased every year since 2002, except for 2009.
The number of births in 2010 was about 20 per cent higher than in 2002 and 13 per cent higher than in 2004, according to the Office for National Statistics.
"Pupil numbers and average class size follow similar trends over time. Therefore, the recent and projected population increases are likely to increase demand for teachers and the number of classrooms, making it more challenging for local authorities to keep key stage 1 classes within the legal limit of 30 pupils per class," the study said.
The researchers also point out that a "higher proportion" of KS1 classes are likely to be full to capacity and that there will not be increases in the number of classes in schools. This could result in a reduction in the proportion of parents getting their child into their first choice of school.
In a ranking of 150 factors on educational achievement by Professor John Hattie of the University of Auckland, reducing class size appeared 114th. It had a positive impact, but was behind other factors - including school size, which ranked at 63.
The Class Size and Pupil-Adult Ratios (CSPAR) study, carried out by the University of London's Institute of Education (2000-2003) found that, in Year 6 classes of 25 or less, teacher-pupil contact was more individualised and task-related with a more active role for pupils, compared with classes of 31 or more.
The Deployment and Impact of Support Staff (DISS) study, also by the London Institute of Education, showed that, in smaller classes, pupils were the focus of a teacher's attention. Engagement was also less in larger classes, especially with low-attaining pupils.
Class Size and Education in England Evidence Report (2011). Department for Education