I admit; this episode went over my head a bit.
Still cute, but I don't get it.
The ghost is another one of Japanese horror story much like Hanako in the toilet, Hachishaku-sama and ofc Mary herself.
IIRC the lore is that she's the ghost of a servant who was drown for a missing plate.
So whenever she shows up she'll be counting plates, but always short.
The punchline is that when Mary dons all the funeral garbs, she looks like the people serving lunch in school instead.