There will be larger class sizes for primary and secondary schools from next year if proposed spending cuts go ahead.
The Department of Education and Skills is planning to increase class size because it cannot cut teachers' pay or conditions under the Croke Park Agreement.
If planned cuts proceed, next year class size in primary schools will increase from 27 to 28 pupils per teacher. The sizing in secondary schools will increase from 19 pupils per teacher to 20.
The Department estimates these measures would save up to €75m.
Increasing class sizes would mean schools could lose up to 1,200 teaching posts.
These permanent teachers would not lose their jobs but would be redeployed to schools with increasing enrolment numbers that need extra teachers.
The Croke Park Agreement protects teachers against any forced cuts in jobs or pay conditions.
The ASTI trade union, representing secondary school teachers, is appealing to the department not to increase class sizes.
It says the move will lead to smaller schools having to totally cut science and language subjects from their timetables.
The Teachers' Union of Ireland has warned that increasing class sizes could limit the subjects offered by some schools.
The TUI is also considering mounting a campaign of protest with parents if these cutbacks go ahead.
The Department of Education and Skills has this week written to various school patrons seeking expressions of interest in operating 14 new schools that it plans to open in 2013 and 2014.
The new schools are being built to cater for a predicted 5% increase in pupil numbers.
The department plans to open schools in Claregalway, Lusk, Naas and Navan in 2013.
While in 2014, it will open schools in Ashbourne, Blanchardstown, Mulhuddart, Drogheda, Dundalk, Greystones and Maynooth, as well as Gaelscoils in Cork city, and Balbriggan and Dundrum in Dublin.