Outcomes: Does Class Size Matter?

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Class Size


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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!

4 thoughts on “Outcomes: Does Class Size Matter?

  1. Christine Ro wrote a nice article on Dunbar’s Number last year (https://www.bbc.com/future/article/20191001-dunbars-number-why-we-can-only-maintain-150-relationships#:~:text=The%20theory%20of%20Dunbar's%20number,about%20150%20connections%20at%20once.
    As you highlight, we’ve had confirmatory bias in play for decades by decision makers to persuade teachers that class size does not matter, and yet it clearly does. If in a working week, a secondary teacher is required to have contact with over 100 or so students, they’ll clearly never develop sufficiently personal knowledge about each of their student’s strengths and weaknesses to make the easy, unconscious interventions that teachers with smaller class sizes can manage. For primary school teachers, the scale of the challenge is smaller; here though the quality of contact and knowledge is more important, and the Dunbar number of 15 comes to the fore. The Catch-22 is that as a country we will never be able to afford to drop class size actually in the state sector to reach the norms achieved in the independent sector, which is probably equally as diverse yet seems to meet the Dunbar ratios more often than not. I am not offering any answers here, just highlighting that the evidence has always suggested we need to half class size and until we find a way of affording that, schools won’t be able to be at their most effective for their teachers and children, and that’s really a great pity.

  2. Its probably an economic consideration for keeping certain class sizes. It might also be a case of keeping status quo as moving from an industrial mindset to a dynamic one is daunting to administrators. Systems thinking efforts are required and administrators endowed with decision-making powers would, as I would if I possess the power, use the power to override and hold the change-makers. Administration is for the purpose of supporting the operations rather than managing practitioners mostly by control. And responding to feedback is the most effortful work. Powerful administrators are being lazy. So first, is there too much power vested in administrators? Are people in authority sitting on their positions as ‘Knowers’ instead of being continuous ‘Learners’?

  3. 3 major issues affect teachers in school.
    1. What does the ‘workload’ in each lesson look like?
    2. What does the ‘workload’ for the day look like?
    3. What does the ‘workload’ for the week, month, semester/term look like?
    If I had only 1 lesson of an hour long a day, with a class of 33, I assure you that the lesson, feedback and more general relationships between the class and I would be fab.
    If I have 4 such hours in the day, and there were sufficient breaks in between that enabled me to manage all that nature and nurture throws at me, I’d manage for about 3 days before the inevitable inability to catch up with those that are wriggling off message wears me down.
    International studies show that there seem vast discrepancies between teacher load, and the UK class size for the state sector needs to be better defined by use of median – what actually are teachers facing. Locally, secondary schools have 30+, with almost no breaks built in and a half day to enable PPA time – almost certainly doomed to fail in terms of keeping teachers and classes connected. Just saying… sometimes such actual arrangements are so harsh, that they are beyond even the parody of SLTNewbie.

    1. I concur James. Whilst I’m a big fan of academic research, most research is often conducted in a laboratory settings rather than in actual classrooms. I don’t think class size research really gets down to the nitty-gritty of all the factors that influence teacher workload.

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