25-Year-Old High School Girl, I Wouldn’t Do This with a Kid
A Hentai and School series worth adding to your list.
Once most students are done with high school, they leave and never come back. However, at 25 years old, Hana Natori finds herself in the role of a student once more at her aunt's request. Since Hana's cousin, Kaho Miyoshi, refuses to go to school, her aunt begs Hana to take Kaho's place, since the two of them are practically identical. But as luck would have it, she is recognized by Okito Kanie—an old classmate from her days in high school—who is now a teacher! With her cover blown, Hana assumes Kanie will expose her secret. But when he suddenly kisses her, she soon learns he has other ideas.
Plot summary from MyAnimeList. Data and images via Jikan API.