It comes after a report from Public Health Scotland predicted 79 per cent of teachers will have full vaccine protection – at least two weeks after the second dose – by August 16.
It will be September 25 before all teachers who accepted the offer of a first vaccine (94 per cent of all teachers) will have this protection.
Currently 53 per cent of teachers in Scotland have been given two vaccine doses.
Dr Patrick Roach, general secretary of the NASUWT, said these figures highlighted the need for continued infection prevention control measures.
He said: “The fact that around one in five teachers may not have had the opportunity to gain full vaccine protection by the time schools return further emphasises the need for continued Covid infection measures to be in place in every school when they reopen in order to help keep all staff and pupils safe."
Schools will need "clear guidance and support", Dr Roach said, adding: “Vaccinations cannot be relied upon to prevent the spread of Covid, not least given that pupils and many staff will not have been vaccinated."
It comes after the Scottish Conservatives called for face masks in schools to be scrapped.
“The success of Scotland and the UK’s vaccine scheme means that face masks in schools are no longer a necessity", said shadow education secretary Oliver Mundell.
He said: "We firmly believe the public health data now supports a return to normality in schools by removing the requirement for face masks and ending the need for children to self-isolate for ten days."
A Scottish Government spokesperson said plans and guidance for schools would be set out before the start of the new term.
The spokesperson said: “Our education advisory group will advise ministers on whether, or to what extent, mitigations can be removed. This includes the wearing of face coverings and the self-isolation requirement for young people in education settings who are close contacts of positive cases.
"As part of our wider preparations for the new academic term, we will set out our plans and guidance for schools in advance of the new term.”