ASTI members face a loss of up to €30,000 each if they reject the latest proposals to settle their long-running rows over pay and junior cycle reform.
Secondary teacher pay packets have already been docked a total of €5m-€6m, arising from three days' industrial action in October and November.
If they vote 'no' in their upcoming ballot, individual teachers will be looking at the long-term loss of annual increment worth an average of €1,200 a year.
About 10,000 ASTI members are due an increment in any one year, and the impact of a threatened increment freeze would depend on where they are on the 25-year scale - but for a new entrant the loss would amount to €30,000.
As the new term gets under way today, a cloud of uncertainty hangs over about 500 second-level schools - and 250,000 pupils - where teachers are members of the ASTI.
The Association of Secondary Teachers' Ireland (ASTI), will ballot its members this month on proposals that emerged in discussions in November.
The union leadership is recommending that its 17,000 members reject the package.
If the deal is not accepted, 500 schools face the threat of ongoing disruption, while ASTI members face big losses around pay and promotion.
Any further disruption would cause considerable upset, particularly for third and sixth-year pupils preparing for the oral, practicals and written exams.
The Department of Education is taking a hard-ball approach and has made it clear to ASTI leaders that it is a final offer and, if members don't accept now, there will be long-term consequences.
The package includes salary increases, payment of increments and other benefits, such as improved access to additional working hours, quicker access to permanent jobs, new opportunities for promotion and protection against redundancy. But the department has set strict conditions for increments due in 2016/17.
It wants a lifting of the union ban on working the 33 'Croke Park' productivity hours so that schools can offer their full schedule of parent teacher meetings in the current year.
It is also insisting on ASTI co-operation with junior cycle reforms so that all third years can complete a new-style assessment in English - on which 10pc of marks in the exam rely - before June.
Fewer than half the country's 60,000 third years have done the assessment because of the ASTI refusal to co-operate, and the State Examinations Commission (SEC) is providing another opportunity in the first week after the Easter holidays.
A 'no' vote in the forthcoming ballot would mean there would not be sufficient time left in the school year to allow for these conditions to be met.
If ASTI members were to reject the deal now, and accept it subsequently, they would be too late to get the 2016/17 increment. So members would be a year behind on the pay scale for the rest of their career.
The department ruled out a retrospective recovery of this increment if the union rejects the package now, but accepts it in the future.
This would have serious financial consequences for those on the incremental scale, and on their pensions.
A freezing of an increment means it would take 26 years for teachers to reach the maximum.
Annual increments average €1,200, but at some points of the scale, they can be worth as much as €3,000 or €5,300.
ASTI members would get the other benefits of the package if it were accepted later - but these would not make up for always being one year behind in increments.
The deal on the table combines the Lansdowne Road Agreement (LRA), a top-up increase for newly qualified teachers and assurances around the introduction of the new junior cycle.
The LRA has been accepted by all other public servants, while the other teacher unions have also accepted the deal for new teachers and the junior cycle reforms. Other unions and Government officials are now working on a successor to the LRA.
ASTI's refusal to accept the LRA means its members are already lagging behind teachers in other unions.
They are not receiving half the allowance for supervision and substitution duties, worth €796 this year, while young teachers in the union are also at the loss of an increase of €1,000 being paid from this month.
While the ASTI remains outside the LRA, its members are also excluded from new promotional opportunities. In normal circumstances, ASTI members would be expected to fill about 700 of the 2,500-3,000 promotional posts that will become available this year in primary and post-primary schools.
The rejection of earlier permanency, promotion posts, employment security and improved access to extra hours will have immediate implications for many teachers as teacher allocations are shortly issuing to schools for September 2017.
A Department of Education spokesperson said a decision on acceptance or rejection was a matter for the ASTI membership and said Education Minister Richard Bruton respected the democratic process of balloting that was due to get under way within the ASTI.