Re:Birthday Song - Koi wo Utau Shinigami

That song is an offering from the Reapers - a Requiem - very near the boundary between heaven and hell where one's soul returns to. In that place is a school in which Reapers - beings that collect human souls - are developed and trained. Assembled as "Reaper Cadets" at this school are the protagonist and her peers who were once human. There, all possessing the same aspiration of becoming a Reaper, cadets pair up with one another to study in order to become a fully fledged Reaper.