Negima!

Words Fly Away, Those Written Remain (Verba volant, scriptamanent) (1x19)

Air date: May 11, 2005

Kazumi starts investigating about a classmate named Sayo Aisaka, whom everybody seems to not notice she is present, not even Kazumi herself. It turns out Sayo was a student of Mahora Academy during the 1940s — and is now a ghost.