Outcomes: Does Class Size Matter?

Reading Time: 4 minutes Does class size really matter? There is often a gap between the views of practitioners and the evidence from researchers, policy makers and others when it comes to evidence on the effects of class size As a result of COVID-19, schools will have had to increase or decrease class sizes due to health advice, lack […]

Outcomes: Does Class Size Matter?


In 2010, Ross Morrison McGill founded @TeacherToolkit from a simple Twitter account through which he rapidly became the 'most followed teacher on social media in the UK'. In 2015, he was nominated as one of the '500 Most Influential People in Britain' by The Sunday...
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Does class size really matter?

There is often a gap between the views of practitioners and the evidence from researchers, policy makers and others when it comes to evidence on the effects of class size

As a result of COVID-19, schools will have had to increase or decrease class sizes due to health advice, lack of teaching staff, classroom spaces and so on, but is reducing class size a long-term solution to teaching, and what does the evidence suggest?

Hundreds of thousands of children are taught in classes of more than 30 . This has been true for at least the last decade, although it’s become more common to have larger primary school classes recently.

For clarity, “Class size is defined as the number of pupils in a class with one teacher. Average class size represents the average number of pupils being taught by one teacher classes during a single selected period in each school on the day” ( DfE, 2011 )

The latest statistics on pupils in schools in England as collected (January 2020) suggest the “average class size in all primary schools decreased slightly from 27.1  in 2019 to 27.0 in 2020. The average class size in all secondary schools increased from 21.7 in 2019 to 22.0 in 2020.

Recent data suggests that the average class size is 24.6.

Does size have any impact?

In a new research paper comparing classes of 17 with class sizes of about 23, researchers ask, ‘Do small class sizes improve student achievement in primary and secondary schools?’

Increasing class size is one of the key variables that policymakers can use to control spending on education. All the available evidence points to no or only very small effect sizes, yet teachers will tell you that more pupils equate to more workload and less individual time with students.

In a new paper , published by academics, Peter Blatchford and Anthony Russell at the UCL Institute of Education (IOE), Rethinking Class Size (the 341-page book) unpicks this topic across six themes:

  1. Differentiated teaching is more difficult
  2. Reduced knowledge about pupils
  3. Classroom management more demanding
  4. Reduced amount of activities
  5. Increased demands of marking etc. and,
  6. Increased teacher stress.

Class size and pupil outcomes

Countless research on class size is cited on page 61, that is, on associations between class size and academic outcomes has some very positive about the benefits of smaller class sizes. However, “questionable conclusions” have been raised about the countless studies cited.

The authors write, “The most obvious way of investigating the effect of class size on pupil attainment is to examine the association between class size on the one hand and some measure of pupil academic performance on the other.”

Many teachers will be shocked to read that “studies, surprisingly, tended to find that pupils in larger classes did better than pupils in smaller classes.” Again, the results are “hard to interpret” because the relationship between “the ‘independent variable’ (in this case class size) and the ‘outcome’ (pupil achievement) can be explained by another, confounding factor.” Those factors are:

  1. Relatively poor-attaining pupils tending to be in smaller classes;
  2. Teachers are forced to change their style of teaching in larger classes;
  3. Experienced (and possibly better) teachers are assigned to larger classes.

If you follow PISA surveys closely, “countries and regions performing at the higher end of the attainment chart, like Hong Kong and Shanghai, have relatively large classes and it is therefore concluded that class size cannot be important “(OECD 2012). Interestingly, these countries pay higher teacher salaries. Countries such as South Korea are also cited, but “parental expectations and very high levels of out-of-school tutoring” may also influence our perception of large class sizes achieving better results.

John Hattie, the Sutton Trust and the Campbell Foundation (of which I have summarised ) are also cited. One key conclusion, and very sensible for that matter, which caught my eye was “For a fairer test, we would need also to take into account what teaching and instruction (e.g. reciprocal teaching, feedback, teaching metacognitive strategies, direct instruction) would be appropriate in classes of different sizes.”

I guess at this stage, without this evaluation, it’s hard to know.


As referenced on pages 83-89 in terms of (CSPAR) Class size and pupil–adult ratio research project, “the effects of class size on academic outcomes are clearest with the youngest students in school” and moving a pupil to a larger class is disruptive and “does impact on progress.” For older students, the researchers “believe class size is important for older pupils, but that the effects are not so obvious and not necessarily direct.”

Teaching assistants deployment has “become a key strategic approach”, which might “bring some of the advantages of smaller numbers of pupils to adults while not increasing the numbers of teachers”, but “negative results on the effect of TAs on pupils’ academic progress shows that the employment of more TAs is not an answer to large classes.”

The project also found that there was “more pupil on-task and less off-task behaviour as class sizes decreased,” and “less on-task and more off-task behaviour as class sizes increased. The researchers conclude that these “results are significant because they show that the problem of large classes, especially in the case of older secondary aged pupils… already attaining at lower levels.”

There are few dedicated studies of class size effects on academic attainment. It is worrying how strong conclusions are drawn by so many on the basis of so few studies.

I will return to this paper to summarise the findings on ways in which teaching is affected by class size. In the meantime, you can download all of this research in a 341-page book for free!

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