Primary school pupils are at risk of being sent home due to a growing shortage of substitute teachers, according to the Irish Primary Principals' Network.
The association said that schools were being forced to use unqualified staff or special education teachers to provide cover.
IPPN President Brian O'Doherty said the challenge for principals in securing external substitute cover for teacher absences "isn't new", but the situation is "more pronounced" at the moment because of the circumstances in which they are operating.
Speaking on RTÉ's Today with Claire Byrne, Mr O'Doherty said one measure that was introduced last year when it was anticipated that absence levels would be higher than normal due to Covid-19 was a substitute teacher supply panel.
However, he said not every school has access to one of these and there is insufficient capacity to meet demand.
Mr O'Doherty said another measure was to redeploy a special education teacher to cover the absence of a mainstream one when no external substitute was available.
The school would then be able to "bank those unused substitute hours" and use them at a later date when a substitute was available, which would compensate for the lost special educational teaching time.
It "ensured that there was continuity of teaching time for classes and there was a protection against children with additional needs being further disadvantaged," Mr O'Doherty said, "but unfortunately the facility for schools to do this has been discontinued this year".
He said about 55% of primary school principals are also teachers and therefore cannot step into the breach because they already have teaching commitments.
Mr O'Doherty also said supervision was not a workable or appropriate solution for children of primary school age.
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"So what is a school to do if there are multiple absences, no sub cover available, and the school doesn't have that facility to redeploy?" he asked.
Mr O'Doherty said the IPPN has met the Department of Education to propose and explore possible solutions with a view to increasing substitute capacity within the system and "some progress" has been made.
The Teaching Council has emailed all teachers on the register requesting that anyone who might be in a position to provide substitute cover would make themselves available, he added.
Mr O'Doherty said schools would also typically have used student teachers, but they are not consistently available because they have course commitments.
He added: "But I do know that there has been contact with the colleges to explore what additional capacity they could provide to the system.
"I do understand that there are plans to increase the scope of those supply panels that I mentioned earlier, but if there aren't sufficient teachers available to fill those posts then that's going to be an ongoing difficulty."
'Almost impossible' to find substitute teacher - Drogheda principal
John Weir, Principal of St Mary's Parish Primary School in Drogheda, said it is "almost impossible" to find a substitute teacher.
St Mary's is one of the largest primary schools in the country, with about 1,100 pupils.
Speaking on the same programme, Mr Weir said there were days last week when eight or nine teachers were absent.
"We have a local network of schools here and I know in the month of September alone, out of 35 schools we had almost 300 days where we couldn't source a sub," he said.
Mr Weir said his school is on a supply panel of teachers, but it only works for expected absences and if he were to look at the panel this morning, the next day he can get a sub is 17 November.
The school had been looking at using local retired teachers or students that perhaps do not have lectures on that day, he added, but the last option is to use special education teachers.
"No principal wants to do that, but that is the only thing keeping the system afloat at the moment," Mr Weir said.