The Government's decision to keep England's schools fully open in the final week of term has been questioned by education unions following a rise in infection rates among secondary school-aged children.
Health Secretary Matt Hancock has announced that secondary school pupils in the worst-affected areas of London, Essex and Kent are to be tested for coronavirus following a surge of cases.
But the minister told the Downing Street briefing that the Government wants to keep schools open because it is "right for education and for public health".
The decision to test pupils in parts of England for Covid-19 comes after secondary schools and colleges in Wales were told to move to online learning from Monday "to reduce transmission of coronavirus".
Schools in England were told that they could take an inset day on the last Friday of term so staff could have a "proper break" from identifying potential Covid-19 cases.
School heads have been calling for more flexibility to end in-person teaching early to reduce the risk of pupils and staff having to isolate over Christmas.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: "We have to question why the plan is to mass test children and there is apparently no consideration of moving to remote learning for the last week of term.
"The number of infections in these areas is very similar to the situation in parts of Wales, where the Government there has decided to move to remote learning in secondary schools and colleges from Monday.
"We really do hope that this is not another sign of the Government in Westminster ploughing ahead with its insistence that schools should remain fully open in England come what may."
Paul Whiteman, general secretary of school leaders' union NAHT, said: "The Government is yet to explain why during this emergency testing period public health is best served by schools remaining fully open.
"A very short-term period of home learning while test results are obtained would ensure further transmission does not occur in schools amongst the most affected group."
Speaking at the briefing, Mr Hancock said: "Looking into the detail, the testing results and survey shows us that by far the fastest rise is among secondary school-aged children, 11 to 18 years old, while the rate among adults in London is broadly flat. But we know from experience that a sharp rise in cases in younger people can lead to a rise among more vulnerable age groups later.
"We need to do everything we can to stop the spread among school-aged children in London right now – we must not wait until the review, which will take place on December 16. We need to take targeted action immediately."
The Health Secretary said he had spoken to the leaders of London's councils and the London mayor, and had "decided to put in place an immediate plan for testing all secondary school-aged children in the seven worst affected boroughs of London, in parts of Essex that border London and parts of Kent".
"We are therefore surging mobile testing units and will be working with schools and local authorities to encourage these children and their families to get tested over the coming days," he added.
But Kevin Courtney, joint general secretary of the National Education Union (NEU), has called on the Prime Minister to urgently address other parts of the country where infection rates remain high.
He said: "Case counts would not be so extreme in schools if Government had agreed to the early closing of schools and a take-up in online learning ahead of Christmas and if they had followed our suggestions about secondary schools moving to rota operation.
"The decision to move all secondary teaching online in Wales is a much more robust response to an increasingly worrying situation."