The Haunting of Hillside School (Cabin Creek Mysteries)

It's a new school year in Cabin Creek and the cousins have a new mystery. When Claire and David look out the art room window and see a girl's pale face, they suspect it's a ghost. How else could someone look into a second floor window and just disappear? As the cousins investigate, they find other peculiar clues: piano music lilting through the halls, the aroma of pumpkin pie, and a crooked old painting that looks like the ghost's face. Is their spooky old school house really a haunted house?
Each of the six books in this series feature atmospheric spot art throughout.

This is #4 in the series.