Schools will be forced to drop subjects if proposals to increase class sizes to save money go ahead, it has been warned.
The Association of Secondary Teachers Ireland said the education sector was already seriously underfunded and could not afford further cuts, branding it a nonsense.
Department of Education officials are examining a range of cost-cutting proposals ahead of December's budget, including adding an extra child to each class.
But Pat King, ASTI general secretary, said children would suffer. "At second level, schools will be forced to drop subjects, that's how it affects second level," Mr King said.
"It's not just larger classes, it means principals, teachers will have to say, 'what subject can we do without', and they'll look at subjects like physics, biology, languages."
A spokeswoman for the Department of Education said there was no plan to increase class sizes. "Nothing is decided at this stage and all areas of education are being looked at in the context of budgetary process overall," she said.
But under the planned comprehensive spending review, every Government department must find areas where savings can be made. Under the terms of the Croke Park agreement, teachers' pay cannot be touched, despite accounting for 80% of the Department of Education's budget.
It is understood officials are examining other areas of expenditure to shave from the department's 9 billion euro budget, including increasing the classroom teacher-pupil ratio by one.
But the Teachers Union of Ireland warned increasing class sizes would cause irreparable damage to the education sector, echoing warnings that subjects could be dropped.
The current pupil-teacher ratio is one teacher for every 19 pupils in secondary schools, and one for every 27 in primary schools.