Nicola Sturgeon announced on March 2 those pupils will return on a part-time basis from March 15 – a change from previous plans that would not have seen them back in schools until after Easter.
The move has led to increased workload for teachers preparing for the return at short notice, EIS assistant secretary Andrea Bradley told BBC Scotland’s Sunday Show.
The union accused the Scottish Government of making the surprise announcement in a bid to distract from Nicola Sturgeon’s evidence to the Alex Salmond inquiry the following day.
“We felt the decision seemed to be more of a political one rather than one that was based on sound educational principle," Ms Bradley said.
"It wasn’t one that was taking account of the very real circumstances in which teachers are currently working and have been working, under really quite significant stress and pressure over the last six months or so.”
Ms Bradley added: “Our sense was that the government was looking for a good news story that week and the reopening of schools on this kind of scale was certainly one way of providing that story.”
She added that teachers had previously put effort into preparing for the return of more senior pupils due to take national exams. Ms Bradley said the new announcement left teachers and schools “with a large number of priorities to try to address over a relatively short space of time”.
“Our concern is that by asking teachers to do too much, they are really going to struggle,” Ms Bradley said.
Education secretary John Swinney denied the announcement had been politically motivated.
“We were concerned about the wellbeing of S1 to S3 pupils who might feel excluded from the return to school if we didn’t give them some opportunity for face-to-face learning before the Easter holidays,” he said.
"That was the one and only motivation that the government had.”
Asked about the effect of the change on teachers, Mr Swinney replied: “What’s important I think is that we focus on the needs of young people, and the needs of young people are absolutely central to my consideration of all of these issues.”