To differentiate ourselves from other horror games.
We can have a back story for each and every ghost that appears in the game instead of having generic ghost enemies. This can also work into the story why not all 100 ghosts appears for all characters. For example, one of the teacher is a pervert lolicon, and he only attacks/shows himself female MC.
since we know that there are 100 people that died, we can have 100 stories, sort of like the traditional japanese folk tale https://en.m.wikipedia.org/wiki/Hyakumonogatari_Kaidankai
We can also have the true ending/best ending revealed if you managed to appease/defeat/purify all 100 ghosts.