Three more special schools have announced they are moving to a two-day week on a temporary basis.
Oakwood and Glenveagh in Belfast and Sperrinview in Dungannon have informed parents they will start "blended learning" from next week.
Belfast's Fleming Fulton last week became the first special school to move to two days.
Education Minister Peter Weir previously said that special schools should remain open for all pupils, but The National Association of Head Teachers warned that while special schools' staff would work hard to keep them open, there might be circumstances in which a school would be required to reduce hours.
"That could happen if safe levels of staffing cannot be maintained due to staff illness or absence, vital support from the Education Authority or the Department of Education is unavailable, or vital and irreplaceable support services and therapies that are needed by children and schools are unavailable," it said.
"The minister needs to be clear with parents that some factors will inevitably arise to require a reduction in on-site provision in order to keep children safe."
Attendance data published by the Department of Education showed that a significant number of pupils and staff had not been able to attend special schools since the start of the new term in January. The reduction in numbers, with many parents and staff fearing conditions are unsafe, has made it difficult to remain open full-time.
In a letter to the parents of pupils, Tish McCann, the principal of Oakwood School, said that following a rigorous review of risk assessments, pupils would attend on either Monday and Tuesday or Thursday and Friday from next week.
"We have determined the risks can only be reduced further by decreasing the daily numbers mixing within classes and school transport," she added.
"We continue to do our utmost to keep our school open and our pupils and staff safe.
"We believe that it is a better approach than the situation we are currently in."
Anne Moore, principal of Glenveagh, said the situation was unlikely to improve in the next few weeks.
Pupils have been asked to attend on either Monday and Tuesday or Thursday and Friday on a temporary basis.
"We can reduce the risks of transmission in light of the fact that most pupils are unable to socially distance or wear face coverings," Mrs Moore said.
The school will let pupils who find it "particularly challenging to remain at home" to attend four days a week, as well as the children of key workers who have no alternative.