Houkago no Kaidan 3

A number of terrifying experiences take place in a school! The mysterious incident "The Day You Were There" at a closed-down alma mater, "The Day of the Dead" in which an older sister in despair after losing her only living relative, her younger sister, and a former teacher standing in a school building just before it was demolished. A former teacher stands in a school building just before its demolition. What is the abominable event he had sealed away ?