15-byou dake: I Know I'm falling

Overview

Hirose Kazumi, the school's beauty, is in love with her older sister's boyfriend. When she learned that they are getting married, she hooks up with Yamamura from her class whom she's been having eye contact lately. They become sex buddies with a condition that they're relationship must be a secret from school and that they won't fall in love with each other. Time flies and her sister got married. One day, Yamamura confessed to her which brings their relationship to an end but it was because he's transferring. Kazumi then realises that she likes him so their love becomes mutual. (Source: Summer Rain)

Source: AniList