Smaller class sizes do not reliably result in better student outcomes, a new study has found.

The effect of smaller class size can vary between countries, academic subjects, years, skills, and many factors are likely to play a role, the study, conducted at Michigan State University in the US, has revealed.

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But Professor Spyro Konstantopoulos, one of the study’s authors, said: “Most class-size effects were not different than zero, which suggests that reducing class size does not automatically guarantee improvements in student performance.”

“Many other classroom processes and dynamics factor in and have to work well together to achieve successful outcomes in student learning.”

The study used data from the Trends in International Mathematics and Science Study (Timms) related to eighth-grade pupils collected in 2003, 2007 and 2011 in four European countries - Hungary, Lithuania, Romania and Slovenia, which mandate maximum class sizes.

This means the sample analysed was quite sizeable: 4,277 pupils from 231 classes in 151 schools.

Timms records data on pupil academic ability in maths and science, together with self-reported attitude and interest towards them, and contains information on class sizes.

The results were mixed. Smaller class sizes were associated with some benefits in Romania and Lithuania, but not in Hungary and Slovenia.

In Romania, class size was associated with greater academic achievement in maths, physics, chemistry and Earth science, as well as greater maths enjoyment.

In Lithuania, smaller classes led to greater enjoyment in biology and chemistry, but not higher academic achievement.

The beneficial effects were also only seen in certain years.

The researchers think that smaller class sizes may be associated with greater benefits for pupils in countries with fewer resources.

Professor Konstantopoulos said: “This finding is perhaps due to the fact that class size effects are more likely to be detected in countries with limited school resources where teacher quality is lower on average.”

The debate over the educational value of small class sizes remains unsettled. While some studies have found it leads to benefits for pupils, there are a number of questions still unanswered: for example, what the optimum size is, or the best educational stage to reduce classes.

The additional funding needed is also a difficulty.

According to the UK’s Education Endowment Foundation, evidence suggests that reducing class size does not show particularly large effects until the size is reduced to fewer than 20 or even 15 pupils, a level where a significant benefit is likely to be expensive.