All confirmed contacts of the positive case must remain at home to reduce the possible spread of the virus to family, friends and the wider community, Ceredigion County Council said.
In addition, pupils travelling on the same bus as the confirmed positive case have also been contacted and asked to self-isolate for 14 days.
These pupils come from more than one year group.
Due to a number of staffing challenges, Years 6 to 13 have been asked to remain at home for the remainder of the week, at which time the situation will be reassessed, the council said.
These pupils will be taught remotely.
All parents have been contacted by the school, the council said.
Parents are encouraged to refer their children for a test if they develop any of the following symptoms: a high temperature, a new continuous cough, a loss or change to sense of smell or taste.
Parents should also be aware of other symptoms early on, such as headaches, tiredness and general aches and pains usually associated with the flu, the council said.