There is no set limit to the number of key workers' children that schools can admit as long as it is safe, the Department of Education has said.
The department issued guidance to schools in Northern Ireland on Friday.
Teachers may be shared between schools to enable them to open on Monday, although normal school meals or transport are unlikely to be available.
Schools in Northern Ireland have closed due to coronavirus but should reopen for key workers' children on Monday.
On Friday, Education Minister Peter Weir said only one parent needs to be a key worker for a child to attend school.
Mr Weir previously said children whose parents provide "essential goods and services" should attend school.
He said he recognised some schools may not be ready on Monday.
In a letter to principals, DE's permanent secretary Derek Baker said providing education to the children of key workers was the "most complex" issue facing education as a result of the coronavirus pandemic.
Schools are also expected to admit vulnerable children, including those with statements of special educational needs or mental health needs.
Mr Baker said principals should keep a record of the number of pupils attending their schools from Monday.
But there is no set limit to the number of pupils a school can admit as long as it is safe and in accordance with Public Health Authority (PHA) guidelines on social distancing.
'School leaders should liaise'
Mr Baker said parents should be prepared to transport their children to school and provide them with a packed lunch.
Teachers can also bring their own children to their school if necessary.
"Schools should also consider the needs of other schools in their area," Mr Baker wrote.
"If a school does not have sufficient staffing levels to open, school leaders should liaise to explore whether staff can be shared between schools or children from more than one school should be accommodated on the same site.
"Schools should be aware of PHA advice when determining the number of children allocated to each room."
Schools only have to open during normal school hours but "this will be kept under review", according to Mr Baker.
Nursery schools, nursery units and special schools are also expected to open.
In a separate statement, Mr Weir said there was no "perfect solution" to the situation.
He said the department would monitor the take up of school places by key workers.
"To be clear, both parents and carers do not have to be key workers," he said.
"We will monitor and assess this situation in the days and weeks ahead and the number of schools which will need to remain open in the future will be revised in the light of this information," he said.
Many schools spent Friday contacting all parents of pupils about the arrangements they were putting in place.
Many principals told parents that due to safety measures they could not accommodate any more than 20% of their existing number of pupils.
One school, Primate Dixon Primary in Coalisland, County Tyrone, said it will open at weekends for children of key workers.
In a message to parents, principal Sean Dillon said they would put arrangements in place for pupils early in the week.
"We are extremely grateful to NHS staff for their work on all our behalf, we always want to acknowledge this," he said.
"As a mark of this, we will also be opening Primate Dixon Primary School on Saturdays and Sundays for children of key workers."
However, other schools have said that due to health and safety concerns they cannot open to any pupils on Monday.
The Newry, South Down and Armagh principals group, for instance, said they supported healthcare workers "unreservedly" but needed more time to carry out risk assessments.
They issued a joint letter to parents in their schools on Friday evening.
"With the lack of clarity, without a detailed operational plan from the Department of Education (by close of business today) and without appropriate protective measures in place, we feel we cannot safely provide childcare for the children of key workers on Monday," they said.
"We need adequate time to risk assess in our schools to ensure we can comply with Public Health Authority guidance relating to Personal Protective Equipment and social distancing."
Some principals in Fermanagh have sent similar messages to parents of pupils in their schools.
They also said that the safest place for children was at home until they received the necessary clarity about safety measures.