The events that transpire in this film take place before, during, and after the events in Suicide Club (2001) in order to help show the backstory of everything that took place in that film.
Mitsuko, the name that Noriko takes on, is one of the main characters in the first film.
The poster in the girls' room is of the band Dessert. This was the fictional girl pop band that was a big part of the first film. The poster advertises the song "Write Once", which was the song that Dessert performed in the ending credits of Suicide Club (2001).