Children in third class and above have to wear face masks in school from today.
New Department of Education guidelines state a medical cert is required for an exemption, and those without evidence will be refused entry.
The government is also calling for less indoor socialising among under-12s for the next two weeks, after a sharp increase in Covid cases.
There were over 18,200 among under-18s in the fortnight to Sunday, compared to just over 13,700 in the previous two weeks.
Suzanne Connolly, the chief executive of Barnardos, is concerned about the new requirement for masks;
"It's really important that it's impact on children's educational, and social and emotional development needs to be monitored because particularly young children, they really rely on social cues to learn how to relate to each other."