Re-opening schools to all pupils at the same time is “not safe”, the first minister has said.
Mark Drakeford defended Wales’ gradual re-introduction of classroom lessons after he was criticised by the Welsh Conservatives.
In England all schools go back on 8 March, but some secondary pupils in Wales will not return until after Easter.
Welsh Tory Andrew RT Davies questioned whether schools were a priority.
But Mr Drakeford said schools were his government’s top priority and that it was following scientific advice.
Under 7s in the foundation phase are going back to school this week, with other primary school children and older secondary students due back on 15 March.
Education Minister Kirsty Williams has said her “preference” is for the final age groups, including years seven to nine, to return after the Easter holidays. The summer term starts on 12 April.
The first minister has previously suggested self-catering accommodation could reopen in time for Easter and non-essential retail could resume on 15 March.
At question time in the Senedd, Welsh Tory leader Andrew RT Davies said: “How can you allow, if there’s any headroom going through the month of March, for schoolchildren to remain out of school and be opening up other parts of the economy by lifting the restrictions?”
Mr Drakeford said: “We will return students to schools as fast as it is safe to do so.
“The advice we have is that it would not be safe to do what he is suggesting.
“Now if it is the policy of the Conservative Party in Wales to return children to conditions that are not safe for them or for their staff, then let him say so.
“This government will not do that. We will follow the science, whatever happens elsewhere.
“The science is you must return children in tranches and you must pause between each one.”
UK government scientific advisers have said re-opening schools would raise the R number which measures who quickly coronavirus is spreading.
The Welsh Government’s own advisers have said a “phased and gradual” approach would allow time to monitor the impact of changes “and if necessary to re-impose changes”.