To Us, from Us (2018)

Class 18, the notorious yakuza class in Wutong Middle School received a letter from their future selves one day. The letter accurately predicted what would happen to class 18 during their high school years, and asked the students to complete three impossible missions to save their sick math teacher. To avoid the misfortunes written in the letter, the bad kids called a truce and gradually developed a heartwarming relationship with the Math teacher on their journey to shift the track of history.