Is Kujira no Kora wa Sajou ni Manabu! hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Special · 2018 · 2 episodes · PG-13 - Teens 13 or older
Comedy, Drama, Fantasy, Mystery, School, Shoujo
Two special episodes set in a school setting. Included with the Blu-ray/DVD volumes of Kujira no Kora wa Sajou ni Utau.
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