At least 160,000 post-primary pupils face being shut out of school on Tuesday, February 4, because of a one-day strike by the Teachers' Union of Ireland (TUI) in the campaign over pay equality.
The stoppage involves 19,000 TUI members, including lecturers in colleges of further and adult education, institutes of technology and Technological University Dublin, where students face lecture cancellations.
About 1,100 workplaces will be hit.
Coming just before the General Election, the strike is designed to exert maximum pressure for a deal to end austerity-era two-tier pay scales once and for all.
About half of post-primary schools will definitely be forced to close and the impact on the rest is unclear at this stage.
The 250 community colleges and vocational schools, where the TUI is the sole union representing teachers, will have no option but to shut.
The overwhelming majority of the 96 community and comprehensive schools, where TUI and the Association of Secondary Teachers Ireland (ASTI) share representation, are also likely to close.
The TUI has some members in the 380 schools run, or previously run, by religious groups.
A spokesperson for the Joint Managerial Body, representing this sector, said at this stage it was not possible to gauge the effect of the TUI stoppage on its schools.
Talks on the pay issue have been going on with the Department of Public Expenditure and Reform but the TUI is not happy.
TUI president Seamus Lahart said they had "exhausted every avenue to bring this matter to resolution and have been left with no choice but to take strike action".
He said while progress had been made, teachers employed after January 1, 2011, would still earn some €110,000 less than longer-serving colleagues over the course of a career.
The Irish National Teachers' Organisation (INTO) and ASTI are also involved in the pay campaign but have no plans to strike.
The TUI said there was "no intention to force others not to pass pickets". However, even with ASTI members in schools where they share representation with the TUI passing pickets, serious disruption or closures are inevitable.
ASTI general secretary Kieran Christie said it was actively seeking, within ICTU, a review of the Public Service Stability Agreement (PSSA) leading to pay matters being addressed.
INTO general secretary John Boyle said negotiations on a successor to the PSSA were expected to begin after Easter and INTO was determined to ensure a solution to pay equality issues was funded in that deal.
Meanwhile, up to 500 home helps, who care for the elderly and disabled, may strike the day before the election. February 7 and 14 have been earmarked if members back industrial action in a ballot starting on Monday.
Their union, Siptu, claims some employers have failed to honour an agreement to restore pay cuts imposed during the recession.