The ASTI has warned they will not accept compulsory redundancies on the back of a review of pupil-teacher ratios in schools.
The 17,000 members of the union are at risk of losing their jobs rather than being re-deployed as they are not protected under the Haddington Road Agreement.
Under the agreement the government has said teachers will be protected from losing their jobs if schools are found to be above the 28:1 pupil-teacher ratio, promising to relocate staff from schools who are over quotas.
ASTI members will be balloted for a third time over today and tomorrow, with a result due in mid-December.
"We have made very clear to our members the implications: that if we reject Haddington Road, that it means we are outside of redeployment," said Pat King of the ASTI.
"They understand the implication. This week again, members are reading the literature, reading what's on offer, and will make a decision."
The Education Minister, Ruairi Quinn, has warned that failure by teachers to accept the Haddington Road Agreement will result in compulsory redundancies in up to 30 schools.
"The very simple way to resist redundancies is to vote yes," he said. "It's absolutely straightforward. They chose to take a decision that took them out of the system of protection."