The First Minister said £5 million of funding would be given to councils to allow them to take “whatever steps” necessary to improve air flow in classrooms.
It comes after education secretary Shirley-Anne Somerville said about 2,000 classes needed improved ventilation, at an estimated cost of £4.3m.
This included £1.6m on air filters, £2.4m for mechanical fans and £300,000 for doors to be “undercut to increase air flow”.
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Scottish Conservative leader Douglas Ross branded the latter idea “bonkers”.
He said: “It would be laughable if it wasn’t such a serious issue. Indeed, there is a fire-safety point here.
“A retired firefighter has warned that the doors in a school are essential for holding back heat and smoke should a fire start.”
Mr Ross added: “Bringing in air filters for classrooms is a much more sensible suggestion that every party supports, but the SNP need to get them distributed quickly.
“They have been far too slow to act on ventilation, but we’ve seen throughout the pandemic that schools have fallen down the priority list for this government.
“The First Minister must guarantee that all of the serious ventilation measures – not chopping the ends off classroom doors – will be in place by the time that schools go back after the February holiday.”
Responding during First Minister’s Questions, Ms Sturgeon accused the Tory leader of being “utterly infantile”.
She said: "We’re not requiring local authorities to chop anything off of doors.
"We’re enabling local authorities, guided by health and safety considerations, to take the actions they consider necessary.”
She said schools “are fit for use”, adding: “The Scottish Government continues to take a range of measures to ensure that children and staff working in schools are as safe as it is possible for them to be.
“One of those measures, of course, is one that Douglas Ross – against all logic and most expert evidence – opposes, which is asking staff and pupils in our secondary schools to wear face coverings.”
Ms Sturgeon said there were “a number of things” that could be done to improve indoor ventilation.
She said: “Partly, that can be about air filtration to purify the air, partly that is about ventilation, so mechanical ventilation systems, but also partly, and this is the key point, it’s about taking measures to ensure that the natural flow of air in a room is maximised.
“If you have doors or windows that are not enabling that natural flow of air in a way that you would want it to, then it would strike me as basic common sense you would take measures to rectify that.
“What we’ve done is give additional money to local authorities to allow them to take whatever steps – air filtration systems, mechanical ventilation or basic rectification of the structure of classrooms – to improve the natural flow of air.”
She added: “That strikes me as basic common sense and if Douglas Ross wants to have serious discussions about these matters, then perhaps he could start by making sure it’s a grown-up discussion.”
Mr Ross asked if the Scottish Government had engaged with the fire service about the safety of amending doors.
Ms Sturgeon said that consideration would be for local authorities.
A spokesman for the First Minister was later asked why doors could not simply be propped open.
He said: “It speaks for itself that to improve ventilation, there are basic steps you can take.”
Speaking at Holyrood’s Covid-19 recovery committee earlier, Labour MSP Alex Rowley demanded ministers show evidence for the proposal.
“I just wonder how you come up with that that’s the solution, and how engaged local authorities are?” he asked Deputy First Minister John Swinney.
“Where is the evidence that spending £300,000 cutting the bottom off doors in schools is actually going to be the answer?"
Mr Swinney insisted local authorities were closely involved in developing guidance.