A year six Notre Dame pupil tested positive yesterday.
They had not left the island in the last 14 days and had no known links to positive cases.
They became symptomatic after the last day of term. Contact tracing is underway.
The news comes nearly three weeks after Guernsey relaxed its border rules, meaning that fully vaccinated adults from the Common Travel Area are able to travel into the island without self-isolating or testing. Since then extra requirements have come in for travellers from Jersey as case numbers there have sharply risen.
Director of Public Health Dr Nicola Brink said community seeding was expected as Guernsey adjusts to living with Covid.
'Our robust contact tracing measures, along with our high vaccination rate means we are well placed to minimise onward transmission,' she said.
'It is worth noting that all schools have had additional safety measures in place since the start of the pandemic, including hand sanitising on entry and general good hygiene. We are incredibly grateful to all teaching staff and children across our schools for having adhered to this so responsibly.'
All year three, four, five and six pupils at Notre Dame will be tested as a precaution, due to there having been some activities where all year groups mixed together at the end of term.
Parents of these pupils do not need to contact the helpline to book a test, they will be contacted directly with information on where and when to bring their child for testing.
Pupils in years three to six are asked to self-isolate until they have received a negative test result.
Parents and other household members do not need to self-isolate at present.
Contacts do not need to call Public Health, but should wait to be contacted by them.
Anyone linked to the school that has not heard from Public Health, should carry on as normal, while continuing with good hygiene measures that are now common practice across the community.
Anyone else who is connected with the school and experiencing Covid like symptoms, no matter how mild, should follow the usual procedure of contacting the helpline on 756938 or 756969 to arrange testing.