So why are pupils not being taught if teachers are reporting for work?
Because the problem does not relate to normal classes, but to breaks, and when teachers are absent and their pupils have to be supervised by a colleague.
Teachers normally do these tasks as part of their daily work, but the ASTI has withdrawn the service in pursuit of its 30 per cent pay claim.
Without it, school managers believe they will be sued if pupils are seriously injured. Rather than risk that, they have advised schools not to accept pupils.
How great is the risk that they could be sued?
A high level of compensation is paid out annually when children have been injured on school grounds. All schools are insured, but managers say several serious incidents and subsequent claims would cause their insurance costs to rise dramatically.
Their other worry is that they could be found negligent if they "knowingly" allowed pupils to come to school when there was no proper supervision available. In that case their insurance company might not cover them.
But could schools not make some other arrangement so classes go ahead?
Yes. Some schools have made their own arrangements and facilitated examination students. But the numbers involved are tiny.
Some schools have taken pupils for classes, but made sure they are not on the premises during breaks or if their teacher is sick. These schools claim this approach helps them avoid any potential liability.
So why is my local school not doing that?
Because most secondary schools are accepting the advice of the school managers' association, the Joint Managerial Body. It does not accept that classes can go ahead provided pupils are not around during breaks. It says there could still be a risk for school management, although not all principals see it that way.
So the only reason they are not accepting pupils is because of health and safety worries?
No. The school managers freely admit that another reason they do not want pupils on the premises is because it could cause further industrial relations problems.
They say that if schools made other arrangements the dispute could escalate. "We have to work with the teachers when this is all over," said one principal this week.
Why don't they bring in parents or other individuals to do the supervision?
Because they say this will also cause tensions with the teachers. The ASTI has said any school which brings in outsiders will not get a supervision service from teachers when the dispute ends.
School managers also claim parents or other individuals need to be trained and paid properly, and that this cannot be done at such short notice.
But could something not be done for the Leaving Certificate students?
Yes. The ASTI could agree to provide supervisory cover for those students, while withdrawing it from others. The school managers have asked it to do this but it declined at the weekend.
Some observers have argued that students of 18 do not legally need to be supervised in the same way as other pupils. But the managers say they owe a "duty of care" to all pupils, even if it is less "onerous" in relation to older pupils.
Is supervision a voluntary service for all teachers?
The situation differs between schools. Voluntary secondary schools, mostly owned by the religious, rely on a voluntary service. Comprehensive and community schools are in a similar position.
However, in vocational schools teachers make up their weekly 22 hours by being rostered for a period of supervision, so part of their salary depends on doing this work.
So is supervision likely to remain at the nub of the dispute?
Not necessarily. The ASTI is considering stepping up its action, but this is likely to involve more strikes and less withdrawal of supervision. Also, the Government may provide money in the Budget for a new supervision regime.