CHAPTER D – Indicator D2: What is the student-teacher ratio and how big are classes?


CHAPTER D – Teachers, the learning environment, and the organisation of schools


Indicator D2: What is the student-teacher ratio and how big are classes?


The following document will summarise the D2 indicator by the OECD Report ‘Education at a Glance’. It will cover the student-teacher ratio and class size throughout OECD countries and economies.


There is a difference with regard to class size between public and private primary institutions. At primary level, the average class in OECD countries in 2019 had 21 students in public institutions and 20 in private institutions. On average across the countries examined, there are 15 students for every teacher in primary education and 13 students per teacher in lower secondary education. The average school class has 21 students in primary education and 23 in lower secondary education. Between 2013 and 2019, the average class size remained constant at lower secondary level both in public and private institutions.


Teachers’ salaries, instruction time, class size, and student-teacher ratios have a considerable impact on the level of current expenditure on education through teacher salary costs. The ratio of students to teaching staff is an indicator of how resources for education are allocated. There is evidence that smaller classes may benefit specific groups of students, such as those from disadvantaged backgrounds (Bouguen, Grenet and Gurgand, 2017), however, overall evidence of the effect of class size on student performance is mixed.


Class size is defined as the number of students who are following a common course of study, based on the highest number of common courses (usually compulsory studies), and excluding teaching in subgroups. Student-teacher ratios provide information on the level of teaching resources available in a country relative to its student population, whereas class size measures the average number of students that are grouped together in a classroom. The number of students per class tends to increase between primary and lower secondary education.


Class size in public and private institutions


In most OECD countries, average class sizes do not differ between public and private institutions by more than one student per class at both primary and lower secondary level. Defining a class size that ensures at the same time high attendance, teacher-student interaction, instructor feedback, and student involvement in class may prove challenging.

Class participation is a central aspect of student learning and instructor teaching, and some studies have revealed that a high amount of participation paired with peer-to-peer interaction contributes significantly to critical thinking (Frijters, ten Dam and Rijlaarsdam, 2008).


Source: OECD/UIS/Eurostat (2021), Table D2.1 (


Student-teacher ratios


Student-teacher ratios do not consider the amount of instruction time for students compared to the length of a teacher’s working day, nor how much time teachers spend teaching. There are around as many countries where the ratio is greater in vocational programmes as there are countries where it is lower. Vocational students require more careful supervision as skill training requires both specialised machinery and a greater level of human resources (Astor, Guerra and Van Acker, 2010). At upper secondary level, the student-teacher ratio is greater in public institutions than in private institutions in 17 countries, smaller in public institutions in 15 countries, and similar for both sectors in 4 countries. This mixed pattern in upper secondary education may, in part, reflect differences in the types of programmes offered in public and private institutions.


Teachers’ aides and teaching/research assistants include non-professional personnel or students who support teachers in providing instruction to students. Teaching staff refers to the professional personnel directly involved in teaching to students.


Finally, class size is calculated by dividing the number of students enrolled by the number of classes. The ratio of students to teaching staff is obtained by dividing the number of full-time equivalent students at a given level of education by the number of full-time equivalent teachers at that level and in similar types of institutions. For the ratio of students to teachers to be meaningful, consistent coverage of personnel and enrolment data are needed.


Original text written by OECD

Summarised by Olga Ruiz Pilato