Seven schools in the Cardigan area and five schools in north Pembrokeshire will shut from Monday due to Covid-19.
Ceredigion Council said it has become “increasingly concerned” about the spread of the virus in the area.
It said it believes “a number of super-spreader events” had led to evidence of more cases of community transmission.
The Cardigan schools will close for two weeks. It is not known for how long the schools in Pembrokeshire will be shut.
In a letter, Ceredigion Council said seven schools and two nurseries would be closed until 7 December and pupils will be taught remotely.
The affected schools include:
The Flying Start Nurseries in Cardigan and Aberporth will also shut, along with Cardigan library.
A number of businesses in the area have also said they will be closed due to the spread of cases.
In addition, Pembrokeshire Council confirmed five schools would close:
The authority was not yet able to tell the BBC when the sites would reopen.