Heaven at The End of 4th Floor

A ghost cannot move to their afterlife if they're holding a grudge in this world. While trying to find out why she's still stuck at her high school, ghost girl Yoon longs to have a living friend. One day, a handsome boy named Sean visits her in the utility room, and he willingly becomes her friend. They soon become best of friends, but Yoon finds out that Sean is suspiciously involved in her death.

Source: Spottoon