Secondary teachers belonging to the ASTI union have narrowly voted to reject the government's pay proposals.
Under the plans, over 60,000 public servants who are on lower rates of pay since 2011, would receive average increases of more than €3,000.
53% voted to reject the proposals, with 47% voting to accept.
58% of members voted.
Members of the other secondary teachers union, the TUI, already voted to accept the pay plans, while the primary teachers union, the INTO, has rejected them.
ASTI President Breda Lynch said: “ASTI members remain steadfast in their determination to achieve justice for their lower paid colleagues – that is, equal pay for equal work.
"This latest proposal from the Government represents insufficient progress and does not achieve equal pay. We will continue to pursue the full restoration of pay for our lower paid colleagues.”