Teachers taking part in the third day of industrial action received unwelcome news when the Department of Education confirmed it would be deducting eight days' pay from their final pay packets before Christmas.
The Association of Secondary Teachers, Ireland, reacted angrily and said the decision to deduct the money on December 21st was "designed to intimidate" and would make a solution to the dispute more difficult.
More than 600 schools were closed yesterday when ASTI members withdrew from supervision duties.
Similar action will take place today, with the majority of schools refusing to accept pupils.
Speaking about the decision to deduct pay, Mr Charlie Lennon, ASTI's general secretary, said it would push teachers towards more national strikes.
He said the union was preparing legal action against the Government and would decide tomorrow when this was lodged with the High Court.
"The Government is trying to treat teachers like children, this is pathetic and will have the opposite effect than the one they desire," he said.
He added that deducting pay so shortly before Christmas was particularly offensive to the ASTI's 16,000 members.
Most second-level schools remained closed throughout the State, although a handful in west and north Dublin accepted some exam students for classes.
Mr George O'Callaghan, general secretary of the Joint Managerial Body - which represents most school managers - said the number of pupils involved was very small.
He said school managers were coming under "some pressure" from parents, but the JMB's advice continued to be that without teachers as supervisors, it was not safe for pupils to attend classes.
He said the JMB was seeking a meeting with the ASTI and one issue it hoped to discuss was arrangements for exam students. So far the ASTI has declined to exempt them from its action.
Mr O'Callaghan said many teachers had given exam students extra work on days when classes were cancelled.
He also said many teachers would attempt to catch up on work with their pupils later in the year.