Other mistake: The killer leaves the corpses of the Dean and Parker where they are and walks away, but they later appear in the room at Stanley Hall.
Suggested correction: That is because the killer is moving around the bodies. Just like when Damon is killed. His body is not where it was left when Alicia Witt comes back with the cops.
Other mistake: Why would Natalie get probation? She wasn't driving. It wasn't even her idea.
Suggested correction: Lawyer here - Not a mistake at all. She would be liable as an accessory, under aiding and abetting. The exact crime varies between common law jurisdictions, but she is laughing along when Michelle is speeding and terrorizing the other driver. In common law jurisdictions, that would be viewed as encouragement of Michelle's committing a crime (she explicitly says she didn't tell Michelle to stop), which is a form of abetting, and indeed carries criminal charges.