Head teachers in England and Wales are waiting to see if they will go on strike on 30 November, alongside other public sector workers, over pensions.
A ballot for industrial action by the National Association of Head Teachers (NAHT) closes at midday on Wednesday.
NAHT general secretary Russell Hobby predicts members will vote in favour of action for the first time in the union's 114-year history.
But Mr Hobby said it was not too late for the government to make concessions.
The results of the ballot are expected to be released later on Wednesday afternoon.
If head teachers vote in favour of the strike, their action is likely to close schools, causing disruption to learning for thousands of pupils.
Working parents will also be inconvenienced as they make arrangements for childcare on that Wednesday.
'Make a stand'
The teaching profession says planned government changes to the teacher pension scheme (TPS) will mean teachers working longer, paying more and receiving less when they retire.
Mr Hobby said were many gaps in the changes, which left about half of NAHT members exposed to a large pay cut.
"Even those heads who may be less affected personally by changes to the pension scheme at this stage in their careers feel they have a duty to make a stand for the next generation of their profession.
"We need to focus on the huge impact of a move away from final salary for school leaders (at a time of heavy retirements and unfilled vacancies), we need to think about what is a reasonable age for someone to remain in the classroom and we need to address the impact of higher contributions on people starting out in their career."
The teachers' pension scheme was affordable and reasonable and a decent reward, he added.
Mr Hobby said it was not too late to stop any action going ahead: "It is late in the day, but not too late for both sides to make real headway in this dispute.
"School leaders are not naturally militant and only consider taking action when their legitimate concerns appear ignored."
Teaching staff and support staff will be joining the strike action by public sector workers on 30 November.
Teaching and non-teaching members of the Association of Teachers and Lecturers (ATL) across the UK and members of the National Union of Teachers (NUT) in England and Wales have voted for continuing industrial action over pensions.
The NASUWT is currently balloting more than 230,000 members across the UK for strike action. The results will be known on 18 November.
The other head teachers' union, the Association of School and College Leaders (ASCL), said it would move to a ballot if it felt negotiations had failed or were not progressing satisfactorily.
ASCL would not have time to ballot members in time for strike action on 30 November.