Is SKET Dance: Tettei Bunseki Special hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Special · 2011 · 1 episodes · PG-13 - Teens 13 or older
School, Shounen
Special aired a couple of days before episode 5. It's a mix of explanations, recaps, and a seiyuu interview.
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