More than half of schools in Northern Ireland had teachers absent due to Covid-19 or self-isolation during the first full week of term.
But the impact on schools was uneven with about one in 20 teachers off for Covid-related reasons in post-primaries.
The absence rates were higher across primary and special schools.
The figures for the week from 10-14 January have been compiled by the Department of Education (DE).
They are based on information provided by hundreds of schools across Northern Ireland.
Some schools have reported difficulty getting substitute teachers when staff are off.
Some post-primaries have also reported problems in getting “subject-specific” cover for pupils taking GCSEs, AS and A-Levels.
But the Department of Education workforce survey of schools shows that about two-thirds of teacher absences in schools during the week were covered by a substitute.
The rates of cover were highest in special schools where about eight in ten absences were covered by a substitute on Thursday and Friday.
The department’s figures also do not indicate when staff absences were covered by a teacher already employed in the school.
Education Minister Michelle McIlveen wrote to schools on Thursday and said that more than 150 retired teachers had registered as substitutes following an appeal by the department just before Christmas.
Some student teachers may also be allowed to teach classes unsupervised during school placements this term.
A minority of schools have, however, had to send some classes home for remote learning on specific days due to staff absences.
The department’s data shows that about one in every 13 teachers in primary and special schools was absent for Covid-related reasons during the week from 10-14 January.
The rate of absence for classroom assistants was slightly higher, but had fallen by the end of the week.
However, more than half of primary schools said they had no teachers absent for Covid-related reasons during the first full week of term.
But just under one in 10 primaries said that more than a quarter of their teachers had been absent at some point during the week.
Most post-primary schools said they had been affected by teacher absences.
About one in eight said they had no Covid-related teacher absences during the week.
But just under half of post-primaries said that the number of staff off was relatively minor – with fewer than 5% of teachers absent for Covid-related reasons.
By Friday, only about one in 25 post-primaries reported more than 15% of teachers off because of Covid.
However, the department’s figures do not show how many teachers or staff are off for other reasons, which are not to do with Covid.
That will have increased the rates of staff absence in some schools.
There are just over 1,000 primary, post-primary and special schools in Northern Ireland.
The department’s workforce data is based on information from about 550 schools across all sectors.