Over 50,000 primary school children will this morning go into classes of over 30 pupils, despite a Government commitment to average class size of 20 for all under-nines by 2007.
An INTO survey underlines the extent of the problem in Dublin and its hinterland.
Classes of over 30 are common in schools across the city, and in Wicklow, Kildare and Louth.
The Republic now has the second largest classes in Europe, according to the INTO.
Last night, its general secretary John Carr said learning - and not crowd control - should be the focus of our primary school classrooms.
The extent of the overcrowding is seen in one case study highlighted in the survey.
At Terenure Boys National School in Dublin there is not one class of less than 30 pupils from junior infants to sixth class. In fifth class there are 34 students in each of the two available classes.
In another school in the locality, Pius X Girls School in Terenure, classes of 30 and over are the norm for children from junior infants to third class.
The INTO has highlighted other examples of very large class sizes. These include one class of 38 pupils at Ss Peter and Paul's Boys National School in Baldoyle.
Other schools where the largest class exceeds 30 pupils includes St Brigid's Boys National School, Palmerstown, (35); Ss Mary and Gerard's National School, Enniskerry, Co Wicklow, (33); Scoil Íosagáin National School, Coolgreany, Co Wicklow, (34); Guardian Angels National School, Blackrock, Co Dublin, (32); Ss Peter and Paul's Boys National School, Baldoyle, (38); and St Paul's National School, Ayrfield (33).
Mr Carr said teachers cannot develop individual talents and skills amid such overcrowding.
Average class size in the Republic is 24.5, while the OECD average is 22.
Average class sizes for other EU states are significantly below the Republic. These include Denmark (19), Italy (18), and Luxembourg (15).
Earlier this year an INTO conference heard from one Kildare delegate that he knew of no school in the county with an average class of fewer than 27 pupils.
The Government promised in the Programme for Government to cut average class sizes for children under nine to 20 children for each teacher by 2007.
However, Minister for Education Mary Hanafin has hinted that the timeframe may have to be shifted because of the shortage of teachers and the new priority given to special-needs education.
Department sources deny that the Government has reneged on the commitment.
Instead, they stress that "people are being a bit more realistic about the timetable, given the number of available teachers and the stress on special needs".
The department says it would take at least 2,000 extra teachers to meet the target, and it is not feasible to recruit this number in the time set out in the Programme for Government.
The INTO would like to see 800 new teachers recruited.
Class sizes case studies: two Terenure schools
Terenure Boys National School
6th class 33 and 33
5th class 34 and 34
4th class 31 and 31
3rd class 31 and 31
2nd class 31 and 32
1st class 34 and 30
Senior Infants 33 and 32
Junior Infants 31 and 30
Pius X Girls NS, Terenure
Junior Infants 30 and 30
Senior Infants 30 and 30
1st class 32 and 32
2nd class 30 and 30
3rd class 32 and 32