Small Class Sizes: When They Matter and When They Don't

Ask almost any parent or teacher what they'd change about schools if they could, and smaller class sizes will likely appear near the top of the list. The idea has strong intuitive appeal: fewer pupils means more attention, fewer distractions, and—ideally—better outcomes.

But does this instinct hold up to scrutiny? The answer is yes, with a few important caveats. The research is surprisingly consistent: small class sizes can make a real difference, especially in the early years and primary phase—but their impact depends on when they're introduced, how they're used, and who's teaching.

This article explores what the evidence actually says about class size, why it matters more in some contexts than others, and how schools can use it meaningfully, without treating it as a silver bullet.

Where the Evidence is Strongest

Early Years and Early Primary: A Window That Matters

Study after study shows that class size reductions have the biggest impact on younger pupils. In the early years of schooling, children are developing foundational skills—language, literacy, self-regulation, working memory—that are especially responsive to adult interaction and feedback. Small classes allow teachers to notice more, respond more, and intervene earlier.

The landmark Project STAR study in Tennessee found that reducing class sizes to 13–17 students (compared to the typical 22–25) produced significant and lasting benefits in reading and maths for pupils in Reception through Year 3. These benefits were most pronounced for disadvantaged pupils, but visible across the board.

Likewise, UK-based research led by Blatchford, Bassett, and Brown found that smaller class sizes in Key Stage 1 enabled more individualised instruction, better behaviour management, and improved academic outcomes. Gains were modest in absolute terms but meaningful over time, particularly for children at risk of falling behind.

Project STAR

Study: Tennessee Student/Teacher Achievement Ratio (1985–89)
Sample: 11,600 pupils, randomised
Key finding: Class sizes of 13–17 led to significantly higher achievement through Year 3 Impact: Gains persisted into secondary school and were strongest for disadvantaged children
Source: Krueger (1999), Mosteller (1995)

Why Class Size Matters Less (But Still Can)

By secondary school, the picture becomes more complex. Older pupils are more independent, learning is more specialised, and the teacher's subject expertise becomes more important than the number of pupils in the room.

This is where research often shows diminishing returns. In John Hattie's Visible Learning meta-analysis, reducing class size had an effect size of 0.21—well below more impactful interventions like formative assessment (0.77), feedback (0.70), or teacher-student relationships (0.52). Hattie's conclusion was not that class size doesn't matter, but that it rarely shifts outcomes on its own.

The OECD's PISA data supports this. High-performing systems like China and Singapore operate with relatively large class sizes, yet still achieve top outcomes, largely due to consistent teaching quality and strong curriculum frameworks.

That said, small classes can still offer advantages in certain secondary settings, particularly where classroom dialogue, behaviour management, or student well-being is central. Discussions-rich lessons can flourish more easily with fewer pupils in subjects like English, Art, or Humanities.

Hattie's Meta-Analysis

Study: Hattie, J. (2009) Visible Learning
Scope: 800+ meta-analyses covering millions of students
Effect size for class size reduction: 0.21
Interpretation: Helpful, but less effective than other strategies unless teaching changes Caveat: Class size gains are often indirect—via improved relationships, feedback, or calm

The Trade-Offs: Cost, Staffing, and Priorities

Reducing class sizes is expensive. It means more teachers, more classrooms, and more operational overheads. In many systems, a small drop in class size may deliver less impact than the same investment in teacher training, curriculum development, or targeted support for struggling pupils.

Yet this isn't an either/or decision. Class size reduction, when done purposefully and in the right age groups, can be a foundation on which other strategies are built.

The Education Endowment Foundation (EEF) notes that class size reduction only yields significant gains when teachers adapt their teaching accordingly—using small groups for formative feedback, personal attention, and differentiation.

A Note on Adult Ratios

It's worth distinguishing class size from adult-pupil ratios, especially in early years and primary settings. A class of 20 pupils may feel very different if supported by multiple trained adults. In some international schools, including ISJ, ratios are closer to 5:1 in Early Years and 10:1 in Primary. While not a panacea, such staffing levels enable more responsive teaching and stronger pastoral care.

In these settings, small groups are not about exclusivity—they are about access: access to adult attention, faster feedback, and subtle redirection when things begin to go off track.

EEF Toolkit

Source: Education Endowment Foundation (2021)
Key finding: Reducing class size can improve outcomes—but only when teachers change how they teach
Note: Effects most visible in Early Years and Key Stage 1
Link: EEF Teaching and Learning Toolkit – Class Size

Final Reflection: Less About the Numbers, More About the Conditions

Small class sizes won't guarantee better outcomes. But they create conditions in which good teaching and strong relationships can flourish. Especially in the first years of school, those conditions matter a great deal.

The real case for smaller classes is not about prestige or quiet—it's about timing, attention, and trust. In the right hands, a smaller group gives space for children to be seen, and for teachers to respond, not just react.

Further Reading

• Krueger, A. B. (1999). Experimental Estimates of Education Production Functions. Quarterly Journal of Economics.

• Hattie, J. (2009). Visible Learning: A Synthesis of Over 800 Meta-Analyses Relating to Achievement. Routledge.

• Mosteller, F. (1995). The Tennessee Study of Class Size in the Early School Grades. Future of Children.

• Blatchford, P., Bassett, P., & Brown, P. (2003). Do Low-Attaining and Younger Students Benefit Most from Small Classes? British Educational Research Journal.

• OECD (2019). PISA 2018 Results: What School Life Means for Students' Lives.

• Education Endowment Foundation (2021). Teaching and Learning Toolkit – Class Size. EEF Website