25-Sai no Joshikousei: Kodomo ni wa Oshierarenai Koto Shite Yaru yo

Hana Natori is an earnest, good-natured 25-year-old woman whose cousin refuses to go to high school. Hana takes her cousin's place and attends the high school. On her first day, though, Hana's ruse is discovered by her former high school classmate Okito Kanie — who happens to be the high school's world history teacher.
(Source: Anime News Network)