Secondary school teachers are threatening to stop supervising pupils in a row over pay.
Their union, the ASTI says their colleagues hired after 2010 after being paid less for the same work.
Staff have also warned that they won't offer substitute teachers until the row is resolved.
Members are to be balloted on industrial action.
But ASTI general Secretary Kieran Christie, says the union will continue to discuss the issues with the Department of Education.
