Ok, don't know how much you have done or not of this quest/storyline, so here goes a summary.
- when you go to school at morning, you'll find two students, Martin & Klaus, talking about how one of them lost his phone in a dare, to enter a haunted bus you get at 3AM in front of the park.
- going to the bus at the established time you'll met the ghost (that do nothing) and the banshee, that hits the MC after he gets the phone (a shining spot), then you must battle her.
- to reach the banshee you need to put out the bonfires separating you from her. But to extinguish the fires with holy water, you must first exorcise the other ghost, from whom she gets energy to maintain the fires.
- [SAVE upon entering the bus, so you can easily play through both possible outcomes for this battle]
- you can choose to either forgive (send to heaven) or punish (send to the abyss) the ghosts.
- After you put out all fires and advances on the banshee, you both are teleported to the park, where you discuss with a much weakened banshee and can choose to forgive (she begs you to have sex before going) or punish (buttfucks it straight to hell) her.
- Either way lets the MC can later summon her back for gloryhole blowjobs in the men's restroom (any time of day or night) of the park, though the dialogue and attitude differ (if you were forgiving, she's loving & lusty, while if you punished her, the whole relation is more acrimonious and based on coercion).
I don't know which parts you have already played or not, so i guess that giving it all a rundown might be good, both for you and people stumped or just starting it. Hope it helps.