Binetsu Hime: Zange no Shou
Japanese title: 微熱姫 懺悔の章
Type: OVA
Source: Manga
Episodes: 1
Status: Finished Airing
Aired: Mar 18, 2001
Duration: 29 min
Rating: Rx - Hentai
Studios: Office AO
Producers: Five Ways
Genres: Hentai

By sheer chance, Mikan manages to get a recording of her best friend, Nodoka, and her teacher having sex. After watching it, she decides that she wants to see more. 

(Source: AniDB)