Secondary school class sizes in Ireland are no smaller now than they were 10 years ago, according to the Association of Secondary Teachers.
The union says a new survey has found that more than one-in-six third-year students have experienced class sizes of 30 or more.
It is blaming the situation on a lack of investment, with Ireland ranked 29th out of 30 OECD countries for money spent on second-level students.
ASTI is releasing the new figures ahead of its annual convention next week.
The Labour Party says it is impossible for teenagers to learn in such large classes and is demanding immediate action to address the situation.