The sixth form at a school in Caerphilly have been told they must self-isolate after a student tested positive for Covid-19.
St Martin's School has contacted parents on Wednesday morning and the sixth formers will have to return to remote learning.
Two members of staff, both heads of the sixth form, are also self isolating.
The rest of the school will remain open.
Caerphilly now has one of the worst infection rates in the whole of the UK and went into local lockdown at 6pm on Tuesday.
The Caerphilly borough is the first area in Wales to go back into lockdown.
At its last Estyn inspection last year, the school had 979 pupils on roll, including 132 in its sixth form.
In a letter posted on the school's website, St Martin's headteacher Lee Jarvis said: "We’re aware of concerns regarding a confirmed case in Year 13 of Covid-19 at St Martin’s School. Advice has been given on self-isolation.
"Due to enrolment of Year 12 and 13, as an entire contact group, as a precautionary measure Year 12 and 13 will need to self-isolate for 14 days.
"Work will be provided via Google Classroom during this period of time and codes for classrooms will be issued shortly.
"If any child has symptoms we advise them to get tested. We’re working closely with the NHS Test, Trace and Protect (TTP) team and Environmental Health based in Caerphilly Council to ensure all appropriate measures are in place to protect pupils, staff and the wider community.
"The school remains open and it is not necessary for any other child to self-isolate, stay away from school or to be tested, unless advised to by a TTP team or they develop symptoms of Covid-19."
Mr Jarvis told WalesOnline: "As a precautionary measure, the isolation period for Year 12 and 13 is 14 days from the date when the risk of infection within the positive case amongst the contact group may have happened and therefore the Sixth Form are able to return on the 14th of September.
"In the meantime, work is being set for learners for all of their subjects and I hope that all of the learners and their families stay well during this difficult time."
For details about other schools affected by coronavirus since the start of the new term, read here.
