One hundred pupils at a school on the Llyn are self-isolating.
Gwynedd Council said action had been taken after a Year 9 pupil at Ysgol Botwnnog, near Pwllheli, tested positive for Covid-19 this week.
The spokesperson said: “We can confirm that a Year 9 pupil at Ysgol Botwnnog has returned a positive Covid-19 Lateral Flow Test result and is self-isolating in accordance with Public Health Wales guidance whilst awaiting confirmation of a Covid-19 PCR test result. Seventy-nine pupils within the year bubble and 20 pupils within a transport bubble are also self-isolating whilst the PCR test result for the individual pupil concerned has been confirmed.”