I'm not entirely sure specifically what it's about as I haven't directly experienced it. But generally, if it's based on a true story in high school, it might be about some sort of extraordinary event that happened in a high school setting, like a strange occurrence, a conflict, or a unique relationship. It could involve students, teachers, or the school environment in some significant way.
Sure. There was a story in my high school where a student claimed that his old family heirloom, a pocket watch, went missing. He was really distraught as it had great sentimental value. One day, another student found it in the lost and found. Turns out, it had slipped out of the first student's bag during a rush in the hallway. It was a simple case of misplacement but it felt like a big 'possession' drama at the time.
Yes. The movie draws inspiration from some accounts that were thought to be true. However, in the process of making it into a film, a lot of creative liberties were taken. For example, the way the possession is shown in the movie might be more dramatized than what was originally reported in the real - life events it was based on. But overall, it has its roots in a true story.