Is Hitoriga The Animation hentai?
Yes
Rated Rx (Hentai) on MyAnimeList.

OVA · 2009 · 4 episodes · Rx - Hentai
Hentai
On the train to school one day, Shirakawa Sumire is molested. However, when she confides in teacher Takahashi, the nightmare escalates... (Source: AniDB)
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