Denbighshire County Council said it was working closely with Dinas Bran, in Llangollen, Mid Wales.
All pupils affected must self isolate until October 27.
Children who travel to school on the Arriva 5 and 5c buses, from Wrexham to Llangollen, must also self-isolate.
Parents and carers have been informed and both the local authority and the school have consulted with Public Health Wales.
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Many children from Chirk and Ceiriog Valley are pupils at the school.
A council spokesman said: "Ysgol Dinas Brân remains open for other pupils and the council is working closely with the school, Public Health Wales as well as NHS Wales Test, Trace and Protect to ensure appropriate measures are in place to protect pupils, staff and the wider community.
"Parents and guardians have been advised to remain alert for the main symptoms of coronavirus.
"Advice requires those self-isolating to stay at home, not to go outdoors to exercise, to visit shops, family or friends, or to other public spaces.
"Please do not invite people to your house."
Further formation on Test, Trace, Protect can be found at gov.wales/test-trace-protect-coronavirus