(Reality Breaking Chapter)

Last one I'll do with a Wiki Summary.

A prequel to Meakashi-hen. Shion is sent away to the all-girls school, St. Lucia Academy, for confinement. One day, a male teacher's body was found in the school swimming pool and the first discoverer, Kōsaka Mizuho, is summoned to the chairman's office to report the details. Shion hears rumors about how Mizuho's grandmother is after Mizuho's life and the girl is taking refuge in the school, so Shion approaches the aloof Mizuho.

I haven't read this side story either so I can't tell you much. And unfortunately it only has 3 Chapters... It's incomplete.

Well here it is.
Utsutsukowashi-hen (Incomplete. Only 3 Chapters)
MANGA EXCLUSIVE. NO ANIME VRS.