Below is a general overview of the series concept:

BONG…

Sora woke up on the rooftop, surrounded by motionless students. A girl mid-laugh. A boy tying his shoe. A teacher dropping chalk. All statues.

It was — Sora’s little sister. She wasn’t frozen. A single silver hairpin glowed in her hand. “I found the second hourglass, sis. In your drawer. I’ve been following you this whole time.”

“Big sis… keep living, okay?” she whispered. “I’ll be here. In the frozen seconds between your heartbeats.”

— Gakuen de Jikan yo Tomare 4: The Keeper’s Sacrifice