This means that if students are isolating due to a test or close contact of a confirmed positive case on or before 7 December then you will be expected back in school before the end of term. If you are self isolating from the 8 December onwards then the isolation period will take you into the Christmas holidays. Remember that the isolation period counts from the last known contact of the person or, if they were tested themselves, the day symptoms started or if asymptomatic, the date of the test.

https://www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection