School Days: Magical Heart Kokoro-chan
Genres: Comedy, Ecchi
Type: OVA, Episodes: 1
Aired: Mar 26, 2008
Studio: TNK
Source: Visual novel
Rating: R+ - Mild Nudity
Synopsis:
Originally an April's Fool in 2007, this one episode OVA is a spin-off of the School Days game by 0verflow, and shows Kokoro Katsura, the little sister of one of the main characters, from her other side. A mahou shoujo out to save the world, or something like that.

(Source: AniDB)