Natural: Another
English name: Teacher's Pet
Genres: Romance, Hentai
Type: OVA, Episodes: 2
Aired: Feb 25, 2000 to Jun 25, 2000
Studio: Arms, Triple X
Source: Visual novel
Rating: Rx - Hentai
Synopsis:
The story is about a young girl who falls in love with her teacher who was formally her sister's boyfriend. Although tension builds as he falls for another another female teacher and rumors of him and the student. 

(Source: ANN)