Even if the execution was slightly off, I like the idea Frozen. A normal hollow, while fun, lacks dramatic punch. The Hollow of someone the kids know, now that has dramatic potential.
Former student who died in a car crash?
Former teacher?
Someone's dead little sister / brother / parent / dog / goldfish?
Former janitor?
Pick your favorite.
"It's the fate of all things under the sky,
to grow old and wither and die."