Premieres October 2025. This visual was posted to the official twitter account. Synopsis from AniList:

Announced at the the “Gintama Ato no Matsuri 2023” event: an anime adaptation of a light novel spin-off series of Gintama. Mainly about school-life played by the main series characters, the storyline revolves around class 3-Z’s students and their wacky homeroom teacher Ginpachi-sensei at Gintama High School.