All primary and secondary schools are closed to pupils in Englands, except for vulnerable children and the children of key workers, for the next six weeks, the BBC reports.
But nurseries and early years settings are being told to remain open.
The government closed schools in England with "the heaviest of hearts,'' Cabinet Office minister Michael Gove said, as he confirmed this year's GCSE and A-level exams have been cancelled.
He said Education Secretary Gavin Williamson would make a statement about alternative assessments on Wednesday.
Ministers wanted to ensure grades are as "fair as possible", Gove said.
He said the decision was made after the UK's chief medical officers recommended a move to Covid threat level five.
One head teacher, Neil Strowger from Bohunt School in Hampshire, said he was "bitterly disappointed and quite upset" for students who have suffered so much disruption this year and "fearful" about what it would mean for them.
Another head teacher Evelyn Ford, of Copthall School in north London, said school leaders had been working hard over Christmas and this week to roll out mass testing and now had to shift their focus to provide lessons remotely for the next six weeks.
