Q&A: How does wintry weather affect school closures in Northern Ireland?
More than 100 schools have taken the decision not to open today.
Counties Londonderry and Tyrone are worst affected by the heavy snowfall.
BBC's Education Correspondent Maggie Taggart explains the reasons for closure.
Principals have had to weigh up whether it is safe for the children, teachers and supervisors to make the journey to school and whether they would be able to get home again.
The schools have to apply for an "exceptional closure day".
However, in practical terms, that is usually done some time after the closure has taken place.
If their reason is accepted, then the school will not be penalised and will not have to make up an extra day in the future. There are checklists to help principals make the correct decision.
Reasons for closure
Is the snow so heavy that teachers and other staff cannot get from their homes to school?
If too few teachers turn up, there is a health and safety issue, because children would not be cared for and supervised in the normal way.
Can pupils make the journey to and from school?
The majority of pupils may live in areas affected by heavy snowfall, so they may be snowed in at home. The bus services may be cancelled, so children cannot be transported to school. Pupils who get to school may face serious problems getting home again, if the bad weather continues. If there are very few pupils, there is no point in opening the school.
Will the meals service be available?
Pupils have to be fed when they are in school, so if the canteen staff and the delivery of meals is affected, the school may not be able to open.
Is the heating sufficient?
If the heating is unreliable or damaged or the electricity fails, there could be a health and safety issue. Children must be kept warm and the electricity failure may affect the alarm system, let alone the difficulty of a darkened school.
Would staff and pupils be safe in school?
Heavy snow could make emergency evacuation of a school very difficult for both the staff and children. In the case of a fire, for instance, emergency vehicles may not be able to travel to the school and children could be left standing outside in the cold if the building is evacuated.
If principals have concerns about the welfare of staff and pupils, they are told to make the decision the night before, if possible, and to have a way of communicating the closure news to parents.
The education and library boards say they tell the schools to use special instructions approved by the BBC and the Department of Education in order to inform families that the school will be closed.