Schools in England could face legal action if they try to switch to online learning in the run-up to Christmas.
New powers introduced through the Coronavirus Act allow the government to issue "directions" to headteachers to keep schools open.
If they fail to comply, Education Secretary Gavin Williamson could apply for a High Court injunction forcing them to do so.
It comes as a headteacher in Ware, Hertfordshire, was sent a "minded to direct" letter from schools minister Nick Gibb warning that the emergency powers could be used.
Presdales School had planned to switch to online learning for the final week of term to ensure staff and pupils did not have to self-isolate on Christmas Day, Schools Week reported.
The Department for Education (DfE) said directions under the act would only be used as a "last resort", while a court order would be the "final stage" if they were not followed.
A spokeswoman confirmed a letter was sent to the school, adding: "They have now changed their plans as to what they are doing this term."