The teachers' strike on Tuesday will close at least 45 more second-level schools than originally feared.
The spread of the disruption could bring the number of schools that will not be open to about 400, forcing up to 200,000 pupils to stay at home.
It was known that up to half of post-primary schools - those in the education and training boards (ETB) sector and the overwhelming majority of community and comprehensive schools - would be closed by the one-day stoppage by members of the Teachers' Union of Ireland (TUI). These are schools where the TUI is the sole or main union.
There was uncertainty about the 380 schools in the voluntary secondary sector - those traditionally run by the religious as well as the growing number under the patronage of Educate Together.
Now, the Joint Managerial Body (JMB), which represents school management in that sector, said it was aware of 45 of its 380 schools taking a decision to close. The final figure may be even higher than this.
In most of these schools, the Association of Secondary Teachers' Ireland (ASTI) is the sole or dominant union representing members and it is not engaging in strike action.
However, ASTI has directed its members not to cover for striking TUI colleagues and not to co-operate with any changes in the timetable designed to get around their absence.
The absence of teachers does not only mean lessons have to be cancelled, but has implications for health and safety because of a school's wider responsibility around supervision of students.
Scoil Bride in Clondalkin, Dublin, is among those that have advised parents that they cannot open because of the large number of TUI members on it staff.
The TUI stoppage is part of its campaign to end two-tier pay scales. It will also hit colleges of further and adult education and institutes of technology, and overall 19,000 members and 1,100 workplaces will stop work for the day.
While much progress has been made in restoring cuts in the austerity era, the current arrangements mean teachers employed after January 1, 2011, would still earn €110,000 less than longer-serving colleagues over the course of a career.
About half of that loss would be in the first 10 years of their career when key life choices are made.
The pressure on Government over pay inequality is showing no signs of easing up, with a decision by ASTI last week to ballot members on possible strike action.
Meanwhile, up to 1,300 crèche owners are set to march with staff and parents in a national protest rally over the "worsening childcare crisis" next Wednesday, which will disrupt childcare services.
However, a threatened strike by up to 500 home-helps in Dublin and Wicklow planned for Friday has been deferred.
They say they will mount pickets on February 21 if the HSE does not transfer money to staff as part of a pay restoration agreement.