Coronavirus cases at five separate schools has led to 606 pupils and 11 staff members in Denbighshire being told to self-isolate.
The council said this includes 240 year nine pupils at Rhyl High School, while one member of staff and 240 pupils from year nine and a school bus route at Ysgol Glan Clwyd, St Asaph are also affected.
Another 90 year three pupils and six members of staff at Ysgol Llywelyn in Rhyl have also been told to isolate.
It said the three schools remain open and all affected have been told to self-isolate until 15 October.
Twenty-two Year 6 pupils and two members of staff at Christ the Word Catholic School, Rhyl, will be required to self-isolate until October 16. At Ysgol Bryn Collen, Llangollen, 14 pupils from nursery and reception classes and two members of staff will be required to self-isolate until October 14
The council added it was working closely with the schools, Public Health Wales, as well as NHS Wales Test, Trace and Protect.
It said letters had been sent to parents and guardians on Saturday.
Anyone self-isolating are required to stay at home, not go outdoors to exercise, to visit shops, family or friends, or to other public spaces and should not invite people to their house.
The Welsh Government has said that you should only get a test if you are showing symptoms of the virus.
The symptoms include:
A new continuous cough
A high temperature
Loss of or change to sense of smell or taste.