The 38-year-old star had gone back to his former school in Gyeonggi to film content for his YouTube channel, Season B Season, and asked three students to show him around the campus.
Due to his popularity, he was mobbed in the corridors by about 30 students.
Witnesses also claimed that some of the students were not wearing masks, although Rain was wearing one, and teachers did not control the situation.
A representative from the school, which had given permission for the visit, said: "We ensured that filming took place during lecture hours and Rain was asked to wait in a quarantined room during break hours.
"The teachers also did their best to maintain all Covid-19 guidelines, consistently monitoring students and emphasising sanitation, but it seems there were a few instances in which we were not able to control the students." – The Straits Times/Asia News Network