Schools are reducing or even scrapping "live" online lessons for up to a fortnight because of the burden of carrying out mass testing of students.
Headteachers have said the need to divert resources to test secondary school pupils will cut interactive lessons ahead of the reopening of classrooms from March 8.
Many students – especially those aged 12 to 14 in Years seven, eight and nine – may not return to school until as late as March 19 because of the time taken to test other years, but their online teaching may be curtailed in the meantime.
One secondary school in London wrote to parents last week, telling them to expect all students "back on site by Friday March 19 at the latest". Schools will then break up for the Easter holiday just two weeks later.
To allow for testing to take place, the parents were told that "students not yet attending lessons on site will access all lessons via presentations and pre-recorded content". The school said the move was necessary "to ensure that staff can be used to support with Covid testing and preparations for students' return to school".
Jules White, the headteacher of a 1,500-pupil school in West Sussex and the founder of the Worth Less? grassroots school movement, said he predicted "10 to 14 days of fairly disrupted learning" while testing takes place. He believed members of his campaign group, which represents several thousand headteachers, will be in the same position.
Mr White said: "Teaching will be interrupted and disrupted [during testing]. There are capacity issues and we need 12 to 15 staff to oversee the testing of 300 pupils a day before they can return. My school will try to continue with live [remote] lessons, but it will be tricky."
In a letter to parents, Mr White said: "The logistics of meeting our obligations are considerable." He added that he expected all students to be back in lessons by March 18, meaning it will take 10 days for the staggered return to be completed.
Geoff Barton, the general secretary of the Association of School and College Leaders, said "it would be understandable if some [schools] have to adapt their online programmes given the very significant logistical challenges involved in organising mass testing". But he said any disruption would be "for only a short period of time".
Under the Department for Education plan, all primary school students will return on March 8 for face-to-face teaching and with no requirement for a test, but secondary school students will undergo three lateral flow tests in school. They will then be provided with two tests each week to undertake at home as school continues. The tests are voluntary.
Gavin Williamson, the Education Secretary, has been careful to stress that secondary school students will be back "from March 8" rather than saying all students will be in classrooms "on March 8".
On Sunday, the Government unveiled a scheme for entire families with children at school or college to test themselves. Parents and carers can apply for the free testing kits through a new website that will go live on Monday or else collect them from workplaces or 500 sites around the country.
Mr Williamson said testing family members "will provide yet another layer of reassurance to parents and education staff that schools are as safe as possible".