Follow the Twinkling Blue of the Stars.

One day, Yamato Tsukimachi—who dreamed of becoming a teacher since high school—comes across a familiar name on his class roster: Isshin Komiya, the name of his best friend who fell into a coma during their senior year. Isshin states that he woke up two years ago, did his rehabilitation, and transferred into the class to get a restart on his high school life. After this dreamlike reunion, though now as teacher and student, the two decide to go through their bucket list of things they weren't able to do before they graduated.

(Source: Ichijinsha, translated)

Note: Includes an extra chapter.