Schools are being given a flexible approach to ensure a "safe return", according to Wales' first minister.
He was responding to calls by teaching unions in Wales to suspend face-to-face teaching in response to the new coronavirus variant.
Mark Drakeford said experts would be "looking at all the evidence again early next week".
Unions have called for a national decision on reopening schools rather than leaving it to local councils.
According to local authorities many secondary schools aim to return from 11 January, with some fully open on 6 January.
A joint statement from nine unions called on the Welsh Government to give a "centralised, coherent response" regarding all educational settings "rather than leaving decisions at local levels".
The statement from ASCL Cymru, GMB, NAHT Cymru, NASUWT Cymru, NEU Cymru, Ucac, Unison, Unite and Voice continued: "We are extremely worried that schools will be opening for face-to-face learning from next Monday, whilst Welsh Government continues to gather information about the nature and impact of the new variant of Covid-19...
"We strongly believe that we need to err on the side of caution and ensure, in advance, that we have the medical 'evidence and information' to ensure that any decisions are the correct ones."
The National Education Union Cymru has called for in-person learning to be delayed until at least 18 January.
The NASUWT has also threatened "appropriate action in order to protect members whose safety is put at risk", while head teachers' union NAHT Cymru said it had taken legal action.
But Mr Drakeford said: "We reached an agreement with our local education colleagues that in Wales we will have a phased and flexible return to school."
Prime Minister Boris Johnson said on Sunday parents should send their children to primary school as long as they are open in their area.
Jackie Parker, head of Crickhowell High School in Powys, which reopens for some form years from Wednesday, said "it would have been more sensible to have had a national decision for the time being until the 18th".
She said it would have allowed time to see if cases of Covid had increased over the holiday period.
"People may have been together during the Christmas holiday," she said.
Figures published by Public Health Wales on Sunday showed 56 new deaths from Covid and 4,011 new cases of the virus.
Wales has been in lockdown since 20 December with restrictions on people meeting others on all but Christmas Day when it was limited to another household and a person living alone.
Mr Drakeford said: "There is no evidence that young people get the illness more severely as a result of the variant.
"Our technical advisory group will be looking at all the evidence again early next week.
"And, of course, we will continue to make decisions in the light of the best knowledge, research and information that's available to us at the time," he told BBC Radio Wales' Sunday Supplement.
He also said mass testing in schools would begin as planned this month, in a decision which has been criticised by NAHT Cymru.
"It will allow more children and more teachers to stay safely in the classroom without having to be sent home because another child or another staff member has tested positive," he said.
The joint unions' statement also said the Welsh Government's testing proposals were unworkable for most schools.
"Due to the chaotic and rushed nature of this announcement, the lack of proper guidance, and an absence of appropriate support, the Welsh Government's proposals will be inoperable for most schools and colleges," it said.
The statement continued: "Any suggestion that schools can safely recruit, train and organise a team of suitable volunteers to staff and run testing stations on their premises by an as yet unspecified date in the new term is simply not realistic."
Sian Gwenllian, Plaid Cymru's education spokeswoman, said "parents and teachers need to know what the plan is for the next few weeks".
"We don't really know very much about this new variant in the way that it transmits within the school community," she said.
"And if it is becoming inevitable that schools will have to close, well, an early decision is better for everybody."
Welsh Conservative education spokeswoman Suzy Davies said: "We've had conflicting reports in the press and on social media about the effect of the new variant on younger children and their role in transmitting the disease - complete confusion reigns...
"The Welsh Government hasn't succeeded in reassuring teachers and in some cases parents as well."