The Irish National Teachers Organisation says Ireland is on course to have the most overcrowded classes in Europe.
New figures from the Department of Education reveal that one-in-five primary school pupils are in class sizes of more than 30.
We currently rank second in Europe when it comes to classe sizes.
But General Secretary of the INTO Sheila Nunan said the situation is about to get a lot worse as these figures relate to last year before the budgetary decision to increase the pupil-teacher ratio from 27 to 28.
Ms Nunan said: "This is a snapshot taken before the impact before the deterioration this September, which if you were to take a snapshot after that, would make it even more dire.
"It adds up to say that the situation is going to worsen because if the implementation of the McCarthy report goes ahead then we will see a worsening situation this year and next year."