In this guide
What schools mean when they quote a number
When a school says "our average class size is 18", that figure can mean several different things. It can be the genuine average of register-takings across the school. It can be the average of timetabled class capacity. It can be the median of a heavily skewed distribution where some classes have 8 and others 26. Or it can be the student to teacher ratio dressed up as class size. The fact that international schools rarely publish the underlying data means parents cannot easily tell which definition is being used.
The honest way to think about class size is per subject per year group. A Year 9 maths class might have 22 students. A Year 13 further maths class at the same school might have 4. Averaging these across the whole school produces a number that flatters the larger class and disguises the smaller one. Both are useful data points; the average is not.
The student to teacher ratio trick
The most common manipulation is the student to teacher ratio. A school with 1,200 students and 150 staff will claim a ratio of 1:8. This includes every teaching adult on the payroll: classroom teachers, learning support staff, music specialists, sports coaches, language assistants, librarians sometimes, and senior leadership team members who do not teach. The number looks impressive in marketing material.
The actual experience of a child in the Year 8 history class at that school may be 24 students with one teacher. The ratio of 1:8 has not lied, but it has not described what the child experiences either. When a school quotes a ratio rather than a class size, ask politely for the timetabled class sizes by year group. Schools that decline are signalling something.
Compare class sizes across three schools
Our school compare tool shows reported class size data alongside fees, curriculum and inspection ratings for up to three schools side by side. Helpful when narrowing from a longlist to a shortlist. Our 10 questions to ask piece covers what to probe on a school tour.
Typical class sizes by year group
The numbers below are the genuine averages we see across credible international schools globally. Individual schools may sit above or below.
| Year group | Typical class size | Notes |
|---|---|---|
| Early Years (FS1, FS2) | 14 to 20 | Often with a teaching assistant; effective adult to child ratio of 1:8 to 1:10 |
| Primary (Y1 to Y6) | 18 to 22 | Sometimes with a part-time teaching assistant in Y1 and Y2 |
| Lower secondary (Y7 to Y9) | 20 to 24 | Core subjects largest; languages and music often smaller |
| IGCSE / MYP (Y10, Y11) | 18 to 24 (core), 10 to 18 (options) | Option subjects vary widely depending on take-up |
| IB DP / A-level (Y12, Y13) | 6 to 16 | HL economics and HL chemistry can be 16+; further maths or Italian B can be 2 to 4 |
If a school's quoted average sits materially below these ranges across the board, ask politely how they manage it. Some genuinely smaller schools achieve it through limited enrolment; many are quoting a different metric.
What the research actually says
The single most-cited piece of class size research is the US Tennessee STAR study from the late 1980s, which found significant academic gains for children in classes of 13 to 17 compared with classes of 22 to 26, particularly for younger children and for children from disadvantaged backgrounds. Subsequent reviews by the Education Endowment Foundation and the OECD have broadly confirmed this pattern for early years and primary, while finding the effect much smaller for mainstream secondary.
The honest summary is that class size matters most for children under 8, for children with SEN, and for older students in seminar-style discussion subjects. It matters less for mainstream lower-secondary lessons, where teacher quality and curriculum design dominate. A class of 22 with a strong teacher outperforms a class of 14 with a weak one in almost every study. This does not mean class size is irrelevant; it means it should not be the primary criterion.
Where small classes matter most
Three places. First, in the early years, where the adult to child ratio is the most important driver of language development and social skills. Second, in any classroom containing children with significant additional needs, where smaller classes allow the differentiation that good SEN provision requires. Third, in IB HL and A-level option subjects, where small group discussion is core to the pedagogy. A small classics A-level group of 6 is genuinely better than a group of 18, because the conversation depends on every student participating.
The places where small classes matter less than parents assume are mainstream secondary core subjects (maths, science, English) and primary specialist subjects (music, sport, art). Excellent teachers can deliver well in classes of 24; mediocre teachers cannot deliver well in classes of 14. See our piece on international school teacher quality for the broader picture.
Questions to ask the school
Ask for the timetabled class sizes by year group for the current academic year. A school that provides the data without hesitation has nothing to hide. Ask what the maximum class size is for each year group, written into school policy. Many schools have a published cap (often 24 or 26) and exceptions are rare. Ask what happens when a year group exceeds the cap: do they open a parallel class, increase staffing, or absorb the additional pupils.
Ask about the smallest classes in the school, particularly in Y12 and Y13 options. Schools with several option subjects running at 2 or 3 students are signalling that they preserve subject breadth despite cost. Schools that close any option below 10 are managing costs at the expense of pupil choice. Both are legitimate approaches; you should know which you are joining. For broader context on choosing a school see how to choose an international school.
One final practical note. If your child has identified additional learning needs, ask specifically about the size of the class your child would join, not the school average. Some schools cluster SEN-supported children in smaller classes; others integrate them across larger ones with an in-class learning assistant. Both can work well, but the practical experience differs enormously. The school's published average will not tell you which model applies to your child.
Frequently asked questions
What is a normal class size at an international school?
Primary classes typically run 18 to 22 students. Secondary classes run 18 to 24 in core subjects, 8 to 15 in IB Diploma and A-level options. Schools quoting averages below 15 across all year groups are usually quoting the student to teacher ratio, which is a different number.
Why do schools quote student to teacher ratio instead of class size?
Because the ratio looks smaller. Student to teacher ratio counts every adult teacher in the school against total enrolment. A school can have a 1:8 ratio while running classes of 24. Always ask for actual class sizes by year group.
At what class size does learning suffer?
Research suggests teacher attention degrades above 25 to 28 students in conventional classroom settings. Below 18, the marginal benefit of further reduction is small except for younger children and for SEN provision.
Are smaller classes always better?
For very young children, for children with SEN, and for sixth-form options, yes. For mainstream secondary classes between 18 and 24, teacher quality matters far more than class size.