The First Minister said there are no guarantees children will be able to return to school on January 18.
The Christmas holidays have been extended until January 11, with home learning during “at least” the first week back.
Addressing the Scottish Parliament on Tuesday, Nicola Sturgeon said her intention was to get schools back to normal from January 18 but added “this will be kept under review”.
She said the current plan was a “precautionary approach” needed to give ministers time to assess the impact the new strain of coronavirus could have on schools and education.
She told MSPs: “Keeping schools open has been a priority for the Scottish Government since August and it remains a priority as far as possible.
“However, these recent developments and all aspects of them mean we need to take a precautionary approach and we also need to give ourselves some time to further assess the situation.
“Schools had been due to open on January 5 onwards, no they will reopen on January 5 for children of key workers and for vulnerable children only.
“For all other pupils the school term will start on January 11, however the first week of term, at least, will take place online.
“We will, if at all possible, get schools back on January 18, that has been a priority and continues to be a priority.
‘One of the things the scientists are exploring right now is whether this virus is transmitting more easily to young people.’Nicola Sturgeon
“If that means the rest of us living under more severe restrictions then we will not shy away from saying that as well.”
Sturgeon also added that scientists were still learning about the new strain and whether it spreads faster among children.
She told MSPs: “One of the things the scientists are exploring right now is whether this virus is transmitting more easily to young people.
“Now there is no consensus on that right now and there is no definitive conclusion on that but it is one of the things that is currently being explored.”
Scottish Conservative Holyrood leader Ruth Davidson said parents needed certainty about plans for schools.
She said: “Parents, pupils and teachers deserve as much clarity from this government as is conceivably possible. But they are not getting it.
“This bombshell announcement on Saturday evening forced people to cancel Christmas plans and find ways to balance work and childcare well into the New Year.
“This is having a significant impact on families and for some it will be about being able to work and pay the bills.
“Yet almost a week later, we have had no meaningful updates, just the same vague assurances we heard again today.”
Sturgeon told MSPs tougher lockdown measures are under consideration in Scotland after Covid-19 cases rose by 15% in the past fortnight.
She said the new measures announced on Saturday – cutting festive easing to Christmas Day visits only, a travel ban, the Boxing Day increase in levels and a delay in pupils returning to schools – are needed to counter the risk of “exponential growth” of the new, more infectious variant of the virus.
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