The best mystery I’ve ever read was "The Adventure of the Blue Carbuncle". The author made it so good by including all of the elements of a mystery and using them at just the right times.
Mystery Starter: It was a normal morning at Mountain Ridge and none of the students had arrived yet. After a mandatory staff meeting, all the teachers went to their classrooms to get ready for the first class. When Mr. Howard got to his classroom he was surprised to see that there was a Civil War cannon in it.