Schools could struggle to cope if record Norfolk Covid-19 rates trigger staff becoming sick or having to self-isolate, teaching unions have warned.
With schools back this week, unions fear staff shortages means youngsters might once again have to learn virtually from home.
Education secretary Nadhim Zahawi has outlined new school Covid measures, including the reintroduction of masks and on-site tests for pupils.
But, with Norfolk's Covid-19 case rates now at new record highs, schools are concerned about increased staff sickness and self-isolation.
Almost 200 Norfolk staff joined a Zoom call on bank holiday Monday to discuss the return to school.
And Scott Lyons, district secretary for the Norfolk National Union of Teachers, said he was concerned about the next few weeks.
Mr Zahawi had suggested merging classes, but Mr Lyons said that had been happening already and was far from ideal.
He said: "The last two weeks before Christmas were pretty hellish for staff, because so many were off and they were having to combine classes.
"We have had members saying they were teaching combined classes of 50 children before Christmas, but that they wouldn't be able to keep doing that in January.
"The reality is that's what's likely to happen. People will start to buckle."
Mr Lyons said there had been problems bringing in supply teachers to cover for sickness because they tended to be in older age groups and were concerned about their own health.
And he said schools were also struggling to retain and recruit school support staff.
Mr Lyons said the government should pause Ofsted inspections and use the watchdog's workforce to teach.
Ex-Suffolk head Geoff Barton, general secretary of the Association of School and College Leaders, said: “Schools and colleges desperately want to be able to maintain face-to-face teaching on a consistent basis, but the reality is that if large numbers of staff are absent this will cause disruption which may include having to send home classes or year groups for short periods of time to learn remotely.
"We would like to see a much improved scheme to help schools with the considerable costs of supply cover than the current meagre scheme which has so many conditions attached that it is inaccessible in many circumstances."
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