Friend of the Devil by Stephen Lloyd
Friend of the Devil is set at a posh boarding school on its own island off the coast of Massachusetts. The school, Danforth Putnam, also serves as an orphanage and has for ages. We've got the typical mix of high school kids, nerds, over-achievers, bullies, and staff who range from caring to a little nutty. Into this mix comes Sam, an insurance investigator on the trail of a valuable lost book.
Friend of the Devil is slasher horror, with plenty of dead teenagers and lots of gore to slip in. Sam is kind of a hard-boiled detective not above threatening the kids on campus. Harriet, the school reporter, is also digging around, and they both uncover more than they expect.
You know how some slasher movies and scary and some are lighter, despite the jump scares. This falls in that second camp. It's funny and over the top. You know who the monster's going to target, you know it's going to be...