Furuya-sensei wa An-chan no Mono

An works part-time at a cafe in Kyoto where she witnesses a handsome man proposing to his girlfriend—and being harshly rejected. An is full of compassion and sympathy for the poor guy. The next day, she discovers that the unhappy lover is her new homeroom teacher, Furuya-sensei! An unexpectedly tender relationship develops between the two and An decides: her teacher Furuya is hers!

(Source: Egmont Manga, translated)