One confirmed case of coronavirus at a school in England could result in an entire year group being sent home, according to new government guidelines.
If a case is confirmed, it could mean all the pupils in that "bubble" must isolate for 14 days, the guidance said when it was published on Friday night.
A rota system could also be used to limit the number of pupils in secondary schools in local lockdown zones.
This would be aimed at reducing a person's contacts, helping to limit the spread of the coronavirus and giving enough time at home for symptoms to show up.
The lengthy guidance said: "Schools should ideally operate a rota system that means pupils spend two weeks on-site followed by two weeks at home.
"However, schools can choose to operate a one-week rota (so, five days on-site, followed by nine days at home) if this is necessary for the effective delivery of the curriculum."
It added: "If a case is confirmed, local public health officials will work with the school to take appropriate measures, including asking all members of a pupil's bubble to isolate for 14 days and access remote education while other pupils continue attending."
More from Covid-19
The Department for Education does not specify the exact size of a "bubble", but it may range from a small number of pupils to a whole year group.
The guidance does not apply to primary school children, as scientific evidence indicates they "play a limited role in transmission".
Education Secretary Gavin Williamson said any changes to school attendance "will only ever be an absolute last resort".
But he added: "However, it is important that both government and schools prepare for a worst case scenario, so this framework represents the sensible contingency planning any responsible government would put in place."
Teachers have criticised the announcement and its timing, with millions of pupils expected to return to class next week.
Geoff Barton, general secretary of the Association of School and College Leaders, said: "To wait until the Friday night before most schools return isn't the government's finest moment.
"Obviously, schools haven't had any chance whatsoever to incorporate this into their planning and will now have to revisit the plans they have put in place."
Paul Whiteman, general secretary of school leaders' union NAHT, added: "We'Read More – Source