Monster High: Haunted (2015)

When Spectra Vondergeist goes back to her old ghost school to find out why Monster High is being haunted, the other ghouls use Boogey Sand to transform into ghosts so they can follow her. There, they die-scover that spirits are low because the Principal weighs students down with ghastly rules and detention chains. It's up to the ghouls and their new ghost friends to unbury the mystery of the hauntings and the secrets of the ghost school!