Newly appointed PE teacher Ikumi Sunohara’s elegant features and curvy, athletic body makes her an instant hit with the male students. However, one man gazes at Ikumi with a strange, suggestive stare. The man is Shiro Yasuno, the same twisted creep who trained and turned Ikumi's best friend into a sex slave! He smiles suspiciously at the appearance of his new prey, Ikumi.One night, Ikumi searches the desk of Shiro Yasuno, whom she has suspected for some time, to find clues about her missing best friend. But just as she does, Shiro ambushes her, before quickly restraining her and knocking her unconscious. When Ikumi wakes up, she finds herself in a special "training room" that Shiro had built in an old school building! While trying her best to hide her uneasiness, Ikumi angrily curses out Shiro! Little did she anticipate the real horrors of Shiro's "training" were about to begin... (Source: Coolmic, edited) 
Note: Spin off of the erotic game by Black Lilith.

Wife Stealing: Endless Indecency