Phantoms / J.A. White.

Enhanced descriptions from Syndetics:

Cordelia's last year at Shadow School brings readers back for the finale in the spooky middle grade trilogy by J.A. White, the author of the acclaimed Thickety series and Nightbooks. Cordelia must complete her quest to save the world of the living from the ghosts, and dangerous phantoms, who terrorize them.

It's her final year at Shadow School, and Cordelia Liu isn't ready to move on. Which is funny, considering helping ghosts "move on" is sort of her thing. But Cordelia can't bear to give up the rare ability to see and help ghosts.

A field trip brings Cordelia, Benji, and Agnes face-to-face with a dangerous phantom--a ghost capable of reanimating dead animals--and a team of professional ghost hunters that disappear right after they capture the phantom. Cordelia can't help herself--she needs to know more about the ghost hunters and the phantom.

It'll mean getting Agnes and Benji to join her and the ghost hunters at a strange village called Shady Rest where ghosts live peacefully in their own homes. But soon, Cordelia, Benji, and Agnes realize that Shady Rest is not quite what it seems. Ghosts are disappearing from their homes. Others start to glow and grow frighteningly long necks and extra teeth. And some of them look strangely familiar. Didn't they save these ghosts already?

In the creepy conclusion to the Shadow School trilogy, author J. A. White says a final farewell to this unique world of ghosts and phantoms even though some friends--and fiends--never truly leave you.