I'm wracking my brain trying to remember the name of a book I read way back in grade 3 or 4 (maybe 5?); I know I purchased it through Scholastic book-order, and it takes place at a boarding school/prep school, and had some really nasty kills (I distinctly remember one character being decapitated while after slipping either on or nearby the school's ice hockey rink, and the description of the murder weapon - almost like a restraint that a dog-catcher would use to pick up a stray, some type of metal rope that can be tightened from a distance...)
On the cover was (I always believed anyway) the main character, in some type of "military" outfit, complete with large hat (seemed very Civil War-esque), and he's looking over his right shoulder while standing at attention as a grey or blueish hand is reaching towards his shoulder (the picture is closer to a portrait of the main character, but I'm almost positive you can see just past his shoulder line on the cover).
Anyone have any idea what I'm talking about? I haven't been able to find anything online even close to it...