Scottish education secretary John Swinney has said that schools can't be reopened until the Covid-19 'R' number is "substantially lower".
The Deputy First Minister issued an update on plans to allow pupils and teachers to return across the country, repeating suggestions that a "phased" return was the only likely outcome.
The 'R' number, which was referred to by First Minister Nicola Sturgeon on Thursday, signifies the amount of people on average one infected person passes the virus on to.
The number is thought to have fallen below one, having been at around three at the start of lockdown, but Mr Swinney says they need to see another significant reduction before there is any suggestion of schools returning.
He told the STV Scotland Tonight programme: "There's going to be a very careful management of this issue across the whole of government because one of the key considerations.
"I can't just sit down one day and say the schools are going back. I have to be absolutely mindful of the sciences involved.
"Your viewers will have probably heard the First Minister talking at lunchtime today about the 'R' number, the reproduction number of coronavirus.
"We need to get that substantially lower than it is today before we can even contemplate bringing any pupils to our schools because obviously if we do that it affects parents going back to their works.
"It's a really careful balance to be struck because what we don't want to do is to undermine the really good work that's been done by a commitment of members of the public to reduce the incidence of coronavirus in our society.
"We have to reflect those in our decisions about Scottish education, just as much as they have to be reflected in the business communities as well."
Mr Swinney also repeated suggestions that a "phased" return was most likely with there being little chance of all pupils and teachers returning to the classroom at once.
He added: "We've made no definitive plans when schools will recommence and we've made no changes to the arrangements for the forthcoming period.
"What I can say definitively is that I kind cannot see a situation where all pupils in all schools will come back in one go.
"I just can't see that happening - for all the reasons of social distancing and the gathering together of many pupils in one individual school.
"We couldn't have 2,000 pupils coming back together again in some of our secondary schools in Scotland.
"So there'll have to be a phased return for different groups within individual schools - and the education recovery group is actively looking at what that might look like."
You can find all the latest updates throughout the day on our daily Covid-19 live blog.
