Second-level teachers' pay packets will be down a total of €10m after they strike again on Thursday in the ongoing row over Junior Cert reform.
The teachers will lose a second day's pay in as many months as a result of their stoppage, which will shut more than 720 post-primary schools, forcing almost 350,000 students to stay at home.
The disruption comes as Leaving and Junior Certificate candidates are preparing for their 'mock' exams, and with only weeks to go to the start of the orals and practicals for the State exams.
The Association of Secondary Teachers' Ireland (ASTI) and the Teachers' Union of Ireland (TUI) have also warned of at least one more stoppage in the weeks ahead. No date has yet been announced.
National Parents Council post primary (NPCpp) president Don Myers said they were very disappointed at the way the impasse has been "dragging on".
He said the strike days cause problems for many hard-pressed parents, who have to take time off work and who also lose a day's pay because of the teachers' action. He said on the last strike day, some parents of children with special needs had great difficulties making arrangements for the care of their children, who were unfairly left without school.
The two unions have rejected proposals for teachers to take over some responsibility for grading their students in a new-style Junior Cert, claiming that the introduction of school-based assessment will compromise standards.
The original plan was for the abolition of the traditional June exams with teachers taking full responsibility for assessing their own Junior Cycle students.
In a compromise in November, Education Minister Jan O'Sullivan said she would retain the traditional exams for 60pc of the marks and that teachers would be asked to assess the other 40pc.
But union leaderships rejected this and embarked on strike action on the basis of a mandate given in ballots conducted earlier last year when the original proposal was on the table.
The Department of Education estimates that payroll savings from a one-day stoppage are about €4.8m, and striking teachers have already been docked for the first in the current series of strike days, on December 2.
The figure represents teachers' gross salary and also includes employers' PRSI paid by the State on behalf of teachers, so the net effect on pay packets will be less.
The savings for teachers in the voluntary secondary schools - generally those traditionally run by the religious - and the community and comprehensive sector come to about €3.1m.
The department does not pay teachers in the Education and Training Boards (ETBs) - formerly the VECs - directly, but the estimated savings for those schools are about €1.7m.
The two unions account for about 27,000 of more than 30,000 teachers in the country's 720 second-level schools. Non-unionised teachers may report for work, or indicate a willingness to work, in which case they will be paid, as will other staff such as special needs assistants and secretaries.