The first local cases of COVID-19 transmission within a school were reported on Tuesday.
According to the city of Daejeon, two elementary school students were infected as a confirmed student had been attending the school for three days in late June.
One is a classmate of the first student and authorities say they were regularly in close contact.
The other student is in a different class, but is believed to have shared the same gym as the first student.
All three are fifth graders, which was one of the grades that returned to school last after other grades had restarted.
Quarantine authorities have decided to test all of the school's fifth graders.
The first confirmed student is believed to have come into contact with more than 150 students.
All of them are due to be tested.
The first student was confirmed to have COVID-19 on Monday.
Officials say he caught it off another family member.
The brother of the student, who is in the third grade of middle school, has been confirmed to have the virus as well.
The city of Daejeon has ordered over 100 large private academies and after school venues to close.
The city will also close 14 elementary and middle schools in the surrounding area and resume home schooling for the students until August.
Kim Do-yeon, Arirang News.