About 55% of pupils in Jersey are currently absent from school, the education minister has said.
Senator Tracey Vallois said the number of missing children had created a "challenge" for the "operational ability" of schools.
Last week the States Assembly rejected a proposition to close state-run schools a week early.
Teaching union NASUWT said the government was "failing" to keep staff and pupils safe.
According to government figures, 785 (84.6%) teachers, 958 (87.8%) non-teaching staff, 6,700 (60.4%) primary pupils and 5,071 (26.2%) secondary students were at state-run schools on Monday.
On Monday it was announced new coronavirus cases had been identified in four Jersey schools, while pupils at several others had been asked to learn from home due to staff shortages.
Senator Vallois said senior advisers were "working closely" with head teachers to monitor health and safety in schools.
She said: "The STAC [Scientific and Technical Advisory Cell] advice has been clear, in terms of the limited effectiveness of curtailing the spread of the virus by closing schools, and the increasing evidence of harm by keeping children out of school for longer periods."
The NASUWT has called on the government "to move immediately to blended learning" and for greater safety protections to be "urgently put in place".
In a statement it said: "Given the surge in cases of Covid in Jersey, the NASUWT believes the government is currently failing to adequately meet its responsibilities to keep staff and pupils safe in schools."
General secretary Patrick Roach added: "Ministers must pull their heads from the sand and take action, rather than the constant dithering and delay that is causing more harm with every passing minute.
"The NASUWT will be putting the Jersey Government on notice of their legal responsibilities in respect of the health and safety of our members and will not hesitate to take further action if there is evidence that the welfare of members is being compromised."