17,000 secondary school teachers are starting industrial action today.
Members of the ASTI won't attend meetings outside school hours or take on additional duties as part of its dispute over the public sector pay deal.
The union says its members may also opt out of covering for absent colleagues - and won't participate in training for the new Junior Cert.
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However, the ASTI has told its members they face explusion from the union if they don't take part.
Pat King, General Secretary of the ASTI told Newstalk Breakfast about latest developments: