Secondary School union the ASTI has dismissed suggestions strike action by TUI members amounts to election blackmail.
Hundreds of schools are set to close tomorrow due to strike action by the Teachers Union of Ireland over pay parity for new entrants.
Around 19,000 members of the Teachers Union of Ireland are set to take part which includes secondary school teachers and college lecturers.
The Association of Secondary School Teachers in Ireland will ballot its members on industrial action over the same issue.
Its General Secretary, Kieran Christie said action is needed:
Mr Christie said: "I make no apology for being a trade unionist and trade unions always try to do the very best they can for their members, and I can say, in this instance we make no apologies at any point because of the horror of what is being visited upon our profession."
President of the TUI Seamus Lahart is defending holding the picket during an election campaign.
Mr Lahart said: "The incoming parties after the election will begin to form a government, they will set out their stall for the next three or four years. We want voters to be able to ask their politicians, are they in favour of continuing the discrimination, unfair, unequal pay, damaging the provision?"