Evidence for Learning: Reducing class size

Reducing class size

Low impact for very high cost based on very limited evidence
Implementation cost
Evidence strength
Impact (months)

Reducing class size is an approach to managing the ratio between students and teachers, as it is suggested that the range of approaches a teacher can employ and the amount of attention each student will receive will increase as the number of students per teacher becomes smaller.

1. Reducing class size has a small positive impacts of +2 month, on average. The majority of studies examine reductions of 10 students. Small reductions in class size (for example, from 30 to 25 students) are unlikely to be cost-effective relative to other strategies.

2. There is some evidence for additional benefits of smaller class sizes with younger students, so smaller class sizes may be a more effective approach during the early stages of primary school. 

3. Smaller classes only impact upon learning if the reduced numbers allow teachers to teach differently – for example, having higher quality interactions with students or minimising disruption.

4. The gains from smaller class sizes are likely to come from the increased flexibility for organising students and the quality and quantity of feedback the students receive (see feedback).

5. As an alternative to reducing class sizes, it may be possible to change the deployment of staff (both teachers and teaching assistants) so that teachers can work more intensively with smaller groups (see small group tuition).

The average impact for reducing class size is around 2 months additional progress over the course of an academic year. The evidence in this area of very limited, so should be treated with caution.

The key issue appears to be whether the reduction is large enough to permit the teacher to change their teaching approach when working with a smaller class and whether, as a result, the students change their learning behaviours. If no change occurs then, perhaps unsurprisingly, learning is unlikely to improve. When a change in teaching approach does accompany a class size reduction (which appears hard to achieve until classes are smaller than about 20) then benefits on achievement can be identified, in addition to improvements on behaviour and attitudes.

When used in isolation, the reduction of class sizes in Australian schools has demonstrated little effect on student achievement. A 2014 study from Macquarie University used international test data to assess the relationship between class size and achievement. The authors concluded that there is not a strong relationship between class size and learning. A 2005 review article by Professor John Hattie noted that, in Australia, it has been difficult to find studies linking smaller class sizes to increased achievement.

  • Effects are similar for both primary and secondary schools.

  • Impact on reading is higher (+2 months) than mathematics (+1 month).

  • Most studies examine reductions of 8 – 10 students. The impact of studies that examine reducing class sizes by 5 students is smaller, on average.

International research evidence suggests that reducing class size can have positive impacts on student outcomes when implemented with socioeconomically disadvantaged student populations. Some studies also have also found that smaller class sizes in primary schools can have a greater positive impact on disadvantaged students than their peers.

In the UK, there is some indicative evidence to suggest that lower primary school students with lower prior achievement and from lower socioeconomic backgrounds may benefit from small classes, although the class size threshold at which this impact is identifiable varies between literacy and maths, and potentially also geographical area.

The evidence suggests that significant effects of reducing class size are not seen until the number of students has decreased substantial (to fewer than 20 or even 15 students). Crucially, a reduction in class size is only likely to be effective if it permits teachers to change their teaching approach to the extent that this changes the learning behaviours of students. High quality implementation of reducing class size might consider:

  • Additional opportunities to provide feedback on students
  • Time for high quality interaction between students and teachers e.g. modelling approaches closely with students.

When introducing new approaches, schools should consider implementation. For more information see Putting Evidence to Work – A School’s Guide to Implementation.

The costs associated with reducing class sizes are very high, as additional staff would be required to reduce class sizes.

The estimated average salary for a teacher in Australia is $89,800.

This estimate does not consider the potential cost of sourcing facilities to host the additional lessons created through reducing class size. Space to host lessons is therefore a prerequisite to reducing class size, without which the costs are likely to be much higher.

The security of the evidence around reducing class size is rated as very limited. 45 studies were identified. Overall, the topic lost three additional padlocks because:

  • A large percentage of the studies are not randomised controlled trials. While other study designs still give important information about effectiveness of approaches, there is a risk that results are influenced by unknown factors that are not part of the intervention.
  • A large percentage of the studies were not independently evaluated. Evaluations conducted by organisations connected with the approach – for example, commercial providers, typically have larger impacts, which may influence the overall impact of the strand.
  • There is a large amount of unexplained variation between the results included in the topic. All reviews contain some variation in results, which is why it is important to look behind the average. Unexplained variation (or heterogeneity) reduces our certainty in the results in ways that we have been unable to test by looking at how context, methodology or approach is influencing impact.

As with any evidence review, the Toolkit summarises the average impact of approaches when researched in academic studies. It is important to consider your context and apply your professional judgement when implementing an approach in your setting.

Evidence strength
Number of studies41
Review last updatedJuly 2021