Another suggestion has been made:
A principal unknowingly endangers her school when she tries to clear the name of a former classmate she sent to jail ten years ago.
What say you? Are you wondering how and why she sent him to jail? The word sent bothers me. And what you may not know is that 10 years ago they were seniors in high school and she squealed on him when she heard that he had plans to 'harm' their school. That's how he ended up in jail. Today she's the assistant principal of a very small private school. Do these unknowns make a difference? They do to me but we have to keep our one-liner short so every word must paint a picture.
I'll be out of pocket for several days so put your thinking cap on and help me out here. :)