Shinigami no Ballad.

Synopsis

Eko Miyazaki, a junior-high school student, is troubled that there are bad rumors going around at a local elementary school about a ghost causing problems, and Eko believes that the ghost is her older sister who had been a teacher at the school but recently died. Eko meets Kouta Seto and learns of his ability to see ghosts and a shinigami. She asks for his help to find her sister in the school in order to tell her that she is sorry about saying she hated her right before she died. They sneak into the school at night and eventually Eko's sister appears in her old classroom. Eko is able to apologize to her in tears and her older sister's spirit disappears. Eko is now finally able to smile for the first time since her sister died.

(Source: Wikipedia)