Larry Flanagan, the general secretary of the EIS trade union, was speaking after John Swinney claimed Scotland could be “very significantly” into the final phase of easing lockdown by then.
The Education Secretary announced what opposition politicians branded the “mother and father of ministerial climbdowns” when he said the Scottish Government is now working towards all pupils returning to school five days a week from August 11.
Mr Flanagan said: “We need to be absolutely clear that it would be a fundamental error on the part of the Scottish Government, our employers, parents, or indeed anyone, to believe that Covid-19 will have gone away in August and that it will be business as usual for schools. It will not be.
“If that is a politically inconvenient truth for anyone, it nonetheless remains a truth.”
Previous plans from the Scottish Government for blended learning – with young people potentially in school for as little as one day a week in some areas – had been criticised by parents.
Mr Swinney argued the “sustained reduction” in levels of Covid-19 in Scotland has enabled him to announce the change in stance.
He thanked teachers for their efforts to develop blended learning plans for schools, saying there is now a “strong” contingency plan in place if pupils can not return to schools full-time.
Mr Flanagan said teachers have been left frustrated that Mr Swinney’s announcement was made so close to the end of the school year after they had “worked so hard to prepare for a blended learning model for reopening”.
He added: “Teachers, including heads and deputes who will have had to take a lead in many areas, are now heading into a summer where uncertainty will be in the air with a whole set of other planning requiring to be tackled.
“Essentially it is a ‘maybes aye, maybes naw’ scenario as ultimately the decision is dependent on where the level of Covid-19 infection will be in seven weeks’ time.”
The change of approach from the Scottish Government is a “political announcement”, Mr Flanagan said, saying it is not an “agreed outcome” from the Covid Education Recovery Group that was established by ministers.
He also stressed the union’s red lines for teachers to return to school, insisting there must be “demonstrable evidence that the virus is under control”.
The EIS is “absolutely certain that physical distancing between pupils and teachers remains essential”, Mr Flanagan added.
He asked: “Why would it be mandatory to wear a face covering on public transport but not in a classroom?
“Why would a till assistant be protected by a Perspex sheet but not a teacher?”