WHO KILLED HIM!

Young girl Charlotte Warren and some of her schoolmates got stuck in school with their History professor during a storm. During the power outage professor went to get some help. After an hour of waiting Charlotte and her classmates decided to look for the professor. But what she saw next was terrifying. Her history professor was laying on the bathroom floor dead covered in blood, there was a note next to him saying "You're next!". Who killed professor? Was it one of her schoolmates? Or was someone else involved in this? Who should Charlotte trust?