The gates of hundreds of secondary schools will remain locked on Monday. Tens of thousands of students with State exams looming will miss out on vital tuition time. Teachers will forfeit pay. Many parents will take leave to stay at home. And there is little prospect of a resolution to indefinite closures.

So, how and why did the leadership of the Association of Secondary Teachers' Ireland bring the second-level education system to the brink of chaos? The simple answer is the union's refusal earlier this year to work just under one hour of additional work a week.

Across the public sector, nurses, civil servants, clerical officers agreed under the Croke Park public sector pay deal to work additional hours, which total about 450,000 annually.

This is the equivalent of between 12,000 and 13,000 public service posts, according to the Government. Clerical officers, for example, work about 100 hours; replacing these hours would cost hundreds of millions of euro.

Like other teacher union, the ASTI signed up to work an additional 33 working hours under the 2010 public sector pay deal. Many teachers, however, regard these extra hours as “detention for teachers” and undermine time that could be spent on extra-curricular activities or class preparation.

Many ASTI members are understandably angry given that many use their spare time to go the extra mile to support their students and schools. Union members are not alone in resenting these extra working hours. They are not popular with members of the Teachers’ Union of Ireland or the primary-level Irish National Teachers’ Organisation either.

The crucial difference is the leadership of those two unions embarked on a much different negotiation strategy which yielded significant gains for new teachers, as well as greater flexibility on the use of these additional hours.

The ASTI, on the other hand, spurned these talks and marched its troops to the top of the hill. In doing so, it has set about a chain of events which have resulted in indefinite school closures from today onwards. A bitter blame game is now erupting over who is is responsible for school closures on health and safety grounds.

Senior leadership of the ASTI claim the Government and school management bodies have had plenty of warning about its refusal to work supervision and substitution duties. Government officials and school management bodies, however, say the union’s directive that no members should co-operate with contingency plans makes it practically impossible for many schools to open.

The fact that principals and deputy principals are not exempt from the action means it is down to voluntary members of board of management to recruit, train and organise vetting for supervisors.

Another obstacle is the fact that school management bodies say it takes about seven weeks to recruit and train supervisors; the union, however, gave the education system three weeks formal notice. The action and its consequences, however, seem hugely disproportionate to the issue at hand. It is questionable whether members were fully aware of the consequences of its current trajectory. Some feel they were left in the dark over the fact that withdrawing supervision cover would lead to indefinite school closures. Others say the realisation that they could lose pay for an extended period has come as a big shock.

The ASTI’s leadership now seems caught in a trap of its own making. The indefinitely closure of schools is not sustainable, not least because teachers will not be paid for the duration. Pressure is likely to mount – both internally and externally – as the week goes on.

Public sympathy will likely be stretched given this is a dispute over ceasing to work an extra hour a week rather than a more principled stance over new entrant pay.

The ASTI is now left fighting battles on multiple fronts – junior cycle reform, new entrants pay – with little sign of placing any realistic solutions on the table. Resolving any one of these disputes would be a big ask; finding a solution to all of these issues within a short period of time seem a near-impossible task.

The lack of moderate voices in the union has been telling. The leadership – a combination of conservatives opposed to partnership agreements and more radical left-wing members – has raised expectations and brought the union to this point.

The vast majority of ordinary members – understandably angry at pay cuts and other measures – trust that their union’s leadership has embarked on a sound strategy to secure gains for members. That belief will be sorely tested over the coming days.