More than 40 per cent of primary school classrooms contain pupils of different grades, according to figures released by the Department of Education.
The Department said that of 17,042 classrooms, some 7,423 had pupils of different ages in the one room. Such classes mixed first, second, third and fourth class pupils together, although teachers treated them separately during lessons.
The majority of such "multi-grade" classes are based in rural areas, although a substantial number are understood to be in the cities.
While some educationists have expressed concern over putting pupils of different ages in the one class, small rural schools often have no option because of the small number of students and teachers assigned to their school.
In a Dail reply to Ms Mary Upton TD, the Minister for Education, Dr Woods, said the average primary class size in Dublin was 27.1, compared to 25.6 pupils per class in the rest of the State.
The higher average class size in Dublin was explained by the fact that "multi-grade" schools tended to be smaller in terms of numbers, and were concentrated in rural areas, said Dr Woods.
Dr Woods also disclosed the average class size in the early childhood education sector. The average class size for junior infants was 25.4; senior infants 27; first class was 27.4; and second class was 27.4. These figures related only to classes with pupils of the same grade.
The average class size is different from the pupil-teacher ratio (PTR). The former is calculated on the basis of the amount of pupils in each class, whereas the PTR divides the number of pupils by the number of teachers in a school to arrive at a figure.
According to the latest statistics from the OECD, the State has the second worst PTR in the OECD at primary level, only beaten by New Zealand.
In its report, Education At a Glance, released last month, the Department of Education's figures for second level were not included.
The PTR at primary and second level are expected to improve in the next few years because of the Government's decision to create 1,000 new teaching jobs.
Falling demographics will also contribute to the figures dropping.