Shintaisou: Shin
English title: Princess 69: Midnight Gymnastics
Japanese title: 新体操（真）
Alternative titles: Shintaisou (Shin), Shintasou Kari 2, Rhythmic Gymnastics
Type: OVA
Source: Visual novel
Episodes: 2
Status: Finished Airing
Aired: Jun 24, 2005 to Sep 30, 2005
Duration: 29 min per ep
Rating: Rx - Hentai
Studios: AT-2
Producers: Discovery
Licensors: Media Blasters
Genres: Hentai
Themes: School

There's a secret competition called "dark rhythmic gymnastics". The entrees complete their beauty and lewd by their technique more difficult than rhythmic gymnastics. 

Haneoka Fubuki is a little brother-in-low of Haneoka Miku who's been missing. While he is looking for his sister, he finds her at the house of Tomomi, Miku's classmate. Acceding to Tomomi, she is trapped in the dark side of "dark rhythmic gymnastics". In order to help her out, he resolves his mind to become a coach of "dark rhythmic gymnastics", and he gets close to Fujimiya Koyuki who was chosen by Tomomi not knowing his fate... 

(Source: AnimeNfo)