A total of 125,000 primary schoolchildren are in classes of 30 or more, according to new Department of Education statistics.
According to the figures, fewer than 20 per cent of classes at primary level meet the Government target of 19 or fewer pupils.
A quarter of a million children are in classes that range from 20 to 29 pupils - some 60 per cent of all pupils.
The INTO claims the average class size is 29.
The new official figures disclosed in the Department's Key Education Statistics have provoked a strong reaction from the INTO. Its general secretary, Mr John Carr, said last night: "The figures show how far the Government is from meeting one of its education targets in the Programme for Government".
The programme promises continued reduction in the pupil/teacher ratio. It promises to introduce maximum class guidelines to ensure the average class size for children under 9 will be below the international best-practice guideline of 20:1.
At present, only about one in every 10 pupils at primary level are in classes that meet Government targets on class size. The maximum class size in the State is 35.
Mr Carr said: "In the modern primary school, teachers are expected to focus on the individual child. However, that cannot be done in any meaningful way when we have classes of up to 35 pupils.
"We can only pay lip-service to the concept of every child being special."
Educationalists believe young children should be in classes of no more than 20 to allow them maximise their potential. It is also necessary to allow teachers address numeracy, literacy and social problems.
The INTO says the introduction of the new primary curriculum, as well as changing social patterns, is increasing the pressure on teachers. It says teachers confront a range of new problems including:
- More demanding and challenging children.
- More children with special educational needs.
- An increasing number of pupils with English as a second language.
- More behavioural problems in schools.