Fee-charging schools have warned that a withdrawal of state funding could ultimately cost the country more.
The Joint Managerial Body (JMB) said private schools save the state money, after a Government report revealed they have millions more to spend on pupils every year.
General secretary Ferdia Kelly said current funding to Ireland's 55 fee-charging schools, which covers the cost of teachers' salaries, would have to be paid regardless of whether a child attends a private or a free school.
He said: "Should the parents of the 25,600 young people currently being educated in fee-charging schools transfer their children to free education schools, the state will not only have to pay the teachers' salaries, but also the capital costs for buildings, capitation grants for the running of the schools, in addition to extra teachers' salaries as the pupil-teacher ratio in free education schools, from September 2013, will be four points lower than that in fee-charging schools."
The JMB, which represents the management of the fee-charging schools, claimed that an independent study from PWC showed the annual cost to the state for a pupil educated in a fee-charging school was 4,552 euro, compared with 8,035 euro in a public school.
Mr Kelly insisted the fee-charging schools - 32 of which are Catholic, 20 Protestant, two inter-denominational and one Jewish - are very much part of the country's post-primary system.
The Department of Education report revealed that private school students have seven times more funding available for extra-curricular activities than youngsters in public schools. That increases to 10 times more for students of Ireland's most expensive schools, where fees are more than 6,000 euro a year.
The report, published by Education Minister Ruairi Quinn, also said that private schools have an average of 1.48 million euro each annually to spend on extras - such as additional teachers, extra-curricular activities and capital improvements.
In a statement, the department said it published the report to "inform future policy decisions about the nature and extent of exchequer funding provided to fee-charging schools".
State funding to private schools currently covers the cost of the salaries of one teacher for every 21 pupils in these schools, compared with one teacher for every 19 pupils in schools in the free education scheme. From September 2013, this will rise to the state paying for one teacher for every 23 students.