25-sai no Joshikousei Season 1

Natori Hana 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 Kanie Akito - who happens to be the high school's world history teacher.