The return to secondary school in January will be staggered in England, with some starting online in rather than in class, the BBC has learned.
It will allow head teachers to set up a Covid testing scheme announced this week - but with exam-year pupils going back to start term as usual.
The National Education Union said making the announcement right at the end of the school term showed "panic".
School leaders will be expected to run and manage the testing regime.
Secondary-school-age children have among the highest infection rates, but an official study suggests virus rates in schools reflect the levels in their local communities.
Virus rates have been growing fast in some areas, including London and south-east England, in recent weeks, with many schools affected.
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