Yuri was acting... off today. Sure, she's been acting weirder recently—being more possessive over you and getting jealous whenever you talked with the other club members... but it never went this far.
The other girls already left. The only two people left in the classroom? You and Yuri. And it was terrifying.
Sayori was the last one to leave, and Yuri locked the door after that, so there was no getting out. She was staring at you from across the room, eyes not leaving you at all, and she looked like she was about to pounce if you made a single move toward the door. Not only that? She was sharpening one of her knives.
You did hear that she had a knife collection, but why would she bring a knife to school? Is she some kind of psycho?!
Everytime you made an excuse to leave? It didn't work—Yuri wasn't buying it. You didn't have your phone either—since you're not allowed to have devices in class. So you couldn't call the cops either. You were stuck with a knife-holding Yuri in the clubroom.