Principals could face many classrooms without teachers on Thursday as the numbers isolating due to Covid spirals.
There is so much confusion about Covid testing and isolation that teachers' unions cannot predict how many teachers will be able to work.
It comes as secondary teachers' union the ASTI called for a delayed and staggered reopening of schools.
Teachers' unions will be seeking clarity from the public health sector and the Education Minister Norma Foley at a crunch meeting today to discuss the school reopening plan.
The ASTI said its members are "uneasy" about health and safety when schools re-open.
The union is calling for updated risk assessments to be presented prior to schools re-opening, which is due to happen on Thursday.
ASTI president Eamon Dennehy said they would be proposing the delayed and staggered reopening at today's meeting.
Face-to-face teaching with examination classes should be prioritised, the union said.
A statement last night by the ASTI follows a meeting of the union's Standing Committee, which assessed the reopening of schools in the context of the prevalence of Covid.
The union said it was "deeply concerned that the Education Minister may re-open schools without putting in place additional measures necessary to safeguard the health and safety of students and staff".
Teachers' Union of Ireland general secretary Michael Gillespie said: "We don't yet know how many teachers will be able to report for work because they don't know how long to isolate from, how long to isolate to, and whether or not those dates will change.
"There needs to be clarity also on if different rules apply if you are a positive case or a close contact.
"Our members are asking us what 'Day One' (of isolating) is. Is it when they get a positive antigen test, or is it when they get a positive PCR test?
"They also don't know if the isolation period is 10 days, seven days, or five days.
"You could also have a situation where you have a good number of teachers turn up on Thursday, but few students, or vice-versa.
"Then you have to ask if it would be better to turn to some sort of online classes. There are so many questions."
The Irish National Teachers' Organisation (INTO) said it expects to be briefed at the meeting on the review of public health measures in schools.
"In particular we want to see quicker response times and interventions from public health when multiple cases arise in primary schools," INTO general secretary John Boyle said.
"We will be seeking full clarity on the progress of the children's vaccination programme and a fast-tracking of the booster programme for those aged 20-29, who work in crowded settings like schools.
"As Covid-19 is rampant in communities, we want an assurance that Government will do more to ensure that infection levels within primary schools are being closely monitored and taken seriously moving forward and that school principals can rely on public health support when they most need it."
Meanwhile, the number of healthcare staff unable to work due to Covid has not yet reached its peak, according to the Irish Nurses and Midwives Organisation (INMO).
Its general secretary, Phil Ní Sheaghdha, said increasing numbers of patients being admitted to hospitals with Covid will translate into those at work getting infected themselves.
It is now estimated there are 6,000 healthcare staff out on Covid-related leave, up from 3,800 before Christmas.