25-sai no Joshikousei

25-sai no Joshikousei  (25歳の女子高生) is a Japanese manga written and illustrated by Reon Maomi. A short-anime adaptation aired from April 2 to June 18, 2017, with the uncut version aired on AT-X.

Plot
Hana Natori is an earnest, good-natured 25-year-old woman whose cousin refuses to go to high school. Hana decides to takes her cousin's place but the ruse is discovered by her former high school classmate Okito Kanie—who happens to be the high school's world history teacher.

Other Sources

 * MyAnimeList (Manga)
 * MyAnimeList (Anime)