The council said the pupils were isolating after coming into contact with a “visiting member of staff”.
A council spokesman said: “Parents and staff were informed earlier [on Thursday evening] and issued with guidance provided by Public Health Wales.
“There is currently no evidence to suggest transmission within the secondary school and it will remain open.
“NHS Wales Test, Trace, Protect has been informed of the confirmed positive case and relevant pupils have been told to self-isolate for 14 days; and remain alert for any symptoms of the virus.”
Anglesey Council’s chief executive, Annwen Morgan, said: “We are taking these precautionary measures after receiving information that a visiting member of staff has contracted coronavirus.
“We are continuing to work closely with the school and supporting staff during this difficult time.
“Our main concern remains the wellbeing of pupils, staff and wider community.
“We will continue to monitor the situation at Ysgol Syr Thomas Jones to see whether any further action is needed.”
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