Most primary schools are very small. Some two-thirds of primary schools have just 90 students or less, and of these around half are single-teacher schools (unidocentes) for up to 30 students. Together, these small schools enrol around 18% of all primary school children (PEN, 2013). By law, classrooms can accommodate between 20 and 35 students, although multigrade classrooms can have 15 students and single-teacher schools can operate with as little as just one student. The average primary class has 14 students, much less than the OECD average of 21 (PEN, 2013, OECD, 2016a). With 27 students on average, class sizes in secondary schools are almost double those of primary schools and above the OECD average of 23.