The Association of Secondary Teachers of Ireland (ASTI) has begun a new campaign to reduce class sizes in secondary schools.
The teacher's union is planning to lobby politicians in the coming months to try to get them to support the campaign. They are also seeking the support of parent's groups.
"Large classes are a serious cause of concern for parents and teachers. The idea of having 25 plus or 30 plus vibrant adolescents in a classroom is outdated and severely restricts the use of modern teaching and learning methods," said the president of the ASTI, Michael Freeley.
According to recent figures, about 35,000 Junior Cycle students are in classes of more than 30, with up to 90,000 in classes of more than 25.
"In 2002, the independent McGuinness Report on Staffing in Second Level Schools recommended the creation of 1,200 additional classroom teachers. For the past four years this report has been gathering dust," he said.
Mr Freeley welcomed the introduction of additional school posts in areas such as special needs, guidance counselling and English language support, but he said that the effect of the new posts on class sizes had been "negligible" as they were not mainstream teaching posts.
Mr Freeley warned that the problem of large class sizes could impact on Ireland's economy in the future.
"Reducing class sizes is the key to ensuring that our second-level education service holds on to its reputation as being one of the best in the world," he said. "Left unaddressed, large classes could have serious implications for overall student attainment and for our future as a knowledge-based economy."