The ASTI teachers' union has accused the Department of Education of cutting vital services to secondary schools by axing 300 concessionary teaching posts.
However, the Department of Education rejected ASTI's criticism, claiming an improvement in pupil-teacher ratios means such posts should only need to be filled in "exceptional circumstances".
The union says the move will affect more than 500 schools and mean bigger class sizes and fewer subject choices.
Concessionary teachers are employed by schools to teach minority subjects and to fill gaps in the school roster. In many cases, they are employed to teach children with special needs.
The Department is understood to have written to schools telling them they will have to employ fewer teachers when schools reopen in the Autumn. The reduction in staffing levels are part of cutbacks outlined Mr Charlie McCreevy's budget.
Last year there were 1,000 concessionary posts in secondary schools across the State; this year there will be 700.
The news of the cuts has been met with anger by school managers and union officials.
The deputy general secretary of ASTI, Mr John White, told ireland.comthe cuts would result in "a diminution of the service offered to students" around the State.
"These cutbacks will affect everything a school offers. There will be larger class sizes as a result and teachers may have to teach ordinary and higher level students together. It could even mean a narrowing of the range of subjects being offered," he said.
Union representatives and school managers' bodies are to meet Department officials this week to discuss the plan.
A Department of Education spokesperson said the pupil teacher ratio has improved from 16:1 to 13.6:1 over the last six years.
"This very significant improvement in pupil teacher ratio of recent years should mean that schools have less need for concessionary teaching posts, and such curricular concessions should only be needed in exceptional cases.
"In the current year 140 of the 750 second level schools appealed to an independent board against their initial staffing allocations and 65 further posts were allocated as a result."