Fuyukara, Kururu.

"Eh?
What do you mean that Shihon-chan might die?
Eeh? Shihon-chan's head has gone missing?"

It seems Shihon-chan's headless body was found in the meditation room.

Murder isn't unusual, after all, there's at least one bloodthirsty killer at school.
Murdering people is fine and all but it's considered a taboo that must be never violated to decapitate and hide their head because one can never be revived if so. If her head isn't found within two days, she'll really kick the bucket!

A boarding system school is isolated from the outside world, and where the immortal girls live. The time it takes for them to become an adult is the same time it takes for them to deage and become a baby, they grow up until they become an adult once more, and then they deage again⁠—a cycle that's being repeated.
Fuyukara, Kururu. - Play-Asia.com
There's only one condition in order to graduate from school: one needed to be deemed a prodigy by someone observing from the outside. The moment that verdict was handed down, the girl's graduation would be recognized and she would disappear from school.

The immortal girls' peaceful school life would be greatly disturbed by a certain murder case.