Larry Flanagan, general secretary of the Educational Institute of Scotland (EIS), said whether he would support the physical reopening of schools on January 18 was “entirely dependent” on the level of infection in communities.
The existing plan is for schools to reopen with remote learning from January 11, with a return to school buildings on January 18.
From January 11, the children of key workers and vulnerable children will be allowed to return to school in person.
Mr Flanagan told the BBC’s Good Morning Scotland: “At the moment the indicators are that the 18th would be a very unlikely restart date for school buildings. Schools will be open in the sense of remote learning being in place, but we’ll have to keep it under review.”
He added: “There seems to be increasing evidence of the role of young people in the transmission of this new variant.”
“That was evident in Kent and in London when they looked to close schools pre Christmas.
“If we have easier transmission through particularly the teenage population, then reopening schools where there’s no physical distancing amongst young people would be a recipe for disaster, it would simply accelerate the transmission of infection in the wider community.
“Schools can only reopen safely when we have effectively suppressed community infection levels, and at the moment, although there is some lag in the data because of the Christmas/New Year break, the indications seem to be that this new variant has taken hold in Scotland, and we therefore have to face up to this new challenge.”
Schools in England are due to return after the Christmas break earlier, with most set to reopen on January 4.
But Education Secretary Gavin Williamson revealed on Wednesday that some schools in areas with the highest Covid-19 cases will remain closed.
Vulnerable children and the children of key workers will still return to schools as planned in those areas.