WTF ! Wasted 1.5 hours of my life

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I don't understand why you don't understand what is going on, it is a pretty easy movie to follow. It does contain some mild spoilers.

A teacher, who is sick and is dying, throws a reunion party for the class that she taught when she was younger. Instead of all the classmates coming only a few of them show up. As the party goes on, we discover that the teacher was very physically and mentally abusive towards a handful of kids in her class. The only people who show up to the reunion are the people who she was most abusive to when they were kids. They all come to search for answers to why the teacher was so cruel and why she screwed up their lives (ex. one student she called fat so she spent the rest of her life in surgery and even though she is skinny she still believes she is fat, another student fell during a track meet and she forced him to do squats for a whole day so he was unable to walk the same again, the teacher was very physical towards one student who may have been molested, etc). However, the teacher has no recollection of doing all the abusive things to them. Looking for answers, they get none from the teacher, so they take drastic measures, almost killing the teacher. As they remember more from their childhood, they remember the deformed child that she locked in the basement, the teacher's son. As kids, all of them were very cruel towards him, except one, the student who was also picked on by the other children. As the night progresses, one by one, the reunion attendees are taken and tortured by a person wearing a bunny mask. The same bunny mask worn by deformed child that was locked in the basement. Could it be the deformed child taking revenge on the kids who made fun of him in the past?