The SSTA union says staffing problems are expected to grow worse in the new year.
The reopening of Scottish schools after the Christmas holidays should be delayed because of the Omicron variant, a teaching union has said.
Seamus Searson, general secretary of the Scottish Secondary Teachers Association (SSTA), said rising cases were already leading to staffing issues and the problem is expected to grow worse in the new year.
Meanwhile, the head of the EIS teaching union said the Government should be prepared to return schools to remote learning if necessary.
However, the Scottish Government has said it is not considering national school closures.
Both union leaders spoke to the Scotland on Sunday newspaper, urging care over the spread of coronavirus in schools.
Mr Searson told the newspaper: “If the numbers keep going up, and it looks like that is going to be the case for some time to come, then we are not going to be in a fit state to reopen schools as normal in January.
“We’re already hearing of schools that are not fully staffed and parents are keeping their kids off to ensure they don’t catch Covid in the run-up to Christmas.
“The idea that we need to keep schools open at all costs just doesn’t add up.”
Delaying the start of the new term would give teachers more time to prepare for mitigation measures, he said.
Larry Flanagan, general secretary of the EIS union, said further measures to curb coronavirus were needed when schools returned.
Mr Flanagan said: “There is a clear need for strong mitigations once schools reopen after the break, with enhanced cleaning regimes, improvements in ventilation, and the continuation of other measures such as use of face coverings and appropriate physical distancing measures.
“The Scottish Government must be led by the science and act in the best interests of protecting public health.
“If that necessitates schools moving to remote learning for a period, then that is a step that the Government must not be afraid to take.”
A Scottish Government spokesman said: “The Scottish Government is not considering school closures.
“As the First Minister has made clear, protecting the education of children and young people remains a top priority.
“National school closures would only be an absolute last resort.
“Covid-19 protections will be reviewed on a daily basis due to the rapid increase in cases of the Omicron variant.”