Larry Flanagan, General Secretary of teaching union EIS, said teachers were “concerned” because of the unknown impact Omicron will have on schools.
Mr Flanagan called on the Scottish Government to maintain an “open chequebook” in supporting local authorities to fund Covid mitigation measures in schools.
It comes after the NASUWT union demanded that the government provide air-cleaning units to all schools which require them, commit to more testing resources and increase financial support to cover the cost of staff absences.
Mike Corbett, NASUWT National Official in Scotland, said on Tuesday the Scottish Government must do “everything it can” to reduce the risk of staff absences and disruption to education.
There has been “slow progress” on the installation of air purifiers in schools, Mr Flanagan said, adding that the government must do more to ensure Covid mitigation measures are fully implemented in every school.
“It should be an open chequebook essentially to make sure we can keep schools open safely,” he said.
It comes as Dr Christine Tait-Burkard of Edinburgh University said it is “almost inevitable” that the return to schools will cause Covid cases to rise.
“It’s almost without a doubt that, when schools return, our numbers will increase further,” she said.
“The virus is very, very prominent in the population at the moment – the week before last, one in 40 people in Scotland had the virus, and that means as soon as schools return, with such a high transmissibility of Omicron, the numbers will rise further, that is almost inevitable.”
Asked whether this will happen even with mitigations such as mask-wearing and increased ventilation, she said: “Even with mitigations.
"As we’ve seen in the past they help to slow things down, just as they have slowed Omicron down quite successfully, actually, over the general population, but it’s both children returning to school, parents returning to workplaces, which will help the virus spread further than it has before.”
Schools have not yet been significantly affected by the Omicron outbreak, Mr Flanagan said, and many teachers are unsure what to expect.
The sector may see increased levels of absence, in line with other industries. Two schools which closed due to Covid before Christmas did so due to staff absence, Mr Flanagan said.
The risk of coming into contact with pupils who are infectious will be a “big concern” for teachers.
"Their working environment is going to be higher risk than it was before,” he said.
The EIS does not support calls from some areas to reduce self-isolation requirements in a bid to reduce staff absence.
“If teachers are close contacts then there will be higher levels of absence. Two schools that closed in December were both closed because of teacher absence rather than pupils,” Mr Flanagan said.
"We are not in favour of shortening the self-isolation rules, because the rules put in place have been best practice as the thing that would most effectively limit transmission of the virus.
“We would need to see some real evidence that shortening self-isolation rules isn’t compromising health and safety before we would advise members to go along with that.”
He added: “The current guidance is there for a reason, and it shouldn’t be changed just because it makes operational sense.
“That’s taking a risk of the health and safety of school staff and school communities.”
Education Secretary Shirley-Anne Somerville has asked secondary school pupils to take a lateral flow test before returning to school.
Scottish Labour’s health spokesperson Jackie Baillie urged the government on Tuesday to ensure the Covid testing system will be able to cope with increased demand.
"Before any new restrictions can be justified the government must act to ensure our testing system is working properly and demonstrate the evidence that new rules would meet the unique challenges of Omicron,” she said.
“Schools across Scotland are beginning their return, but it is unclear what meaningful steps, such as improving ventilation, have been taken to stop the spread of Covid in the classroom.”
The EIS said it has not had any reports of trouble with access to lateral flow tests.
Deputy First Minister John Swinney insisted the testing system will be able to cope with the return to school, but there may be “congestion” among PCR tests.
“Obviously it’s important that individuals undertake those lateral flow tests, that is a critical part of our of our defence against Omicron,” he said.
“I think what’s encouraging is that the level of lateral flow device testing is high, many individuals are participating. The most recent data I’ve seen indicates that, in the last seven days, 85 per cent of the population took part in lateral flow tests, which is very, very good.
“But obviously, as people return to school, staff, pupils, as people return to the workplace, it’s important that they do that before they embark on it and then, of course, if they’re positive, to take the necessary steps to receive a PCR test.”
Asked about PCR testing capacity, he said: "I think there will be congestion at different points.
"It's impossible for me to give you a guarantee that at individual test sites there will not be challenges in particular geographies, but I'm confident that we have got a strong testing proposition."