The Big Flush: Book 4 — Trina Robbins

Poor Megan -- history's repeating itself. She's been booted from Stepford Prep, and her father is sending her to visit Pine Lake Academy . . . a boarding school. This could mean the end of the Chicagoland Detective Agency! Raf and Bradley come along to get a sniff at the new school, but when Megan ducks into a restroom marked do not enter and Raf takes a sip from an old fountain, school becomes the least of their problems. Something spooky is knocking around in the pipes, and now it has its hooks in them.

Have Raf and Megan really been taken over by a ghost from a hundred years ago? Can Bradley dig up the mystery that's dogged Pine Lake Academy for a century? What deeply buried dastardly deeds will bubble to the surface?