The Class 3 Calamity is a curse that haunts the 9th Grade students of Yomiyama North Middle School's Class 3.
Every year in Yomiyama North Middle School's 9th Grade Class 3, past records are mysteriously altered and an unidentifiable dead person joins class as an extra student. Changing rosters or class assignments fails to solve the issue and eventually, the dead student brings enough bad energy into the classroom resulting in a student and one of their family members dying every month.
No permanent cure has been found, however, one class was able to stop the calamity after it began by using currently unknown methods and acts involving a shrine. After years of death, the students and teachers of Class 3 found a preventive measure with a success rate nearing 50%. The students and teachers simply ignore one student and act as if they don't exist, which figuratively puts the class count back at a normal level, and prevents people from dying.
26 years before the start of the series, star student named Yomiyama Misaki tragically died in an accident. Unable to cope with or accept his death, his classmates, teachers, and students acted as if he was still an active member of the class. After graduation, they noticed that he still appeared in class photographs despite being dead.