Benkyou ni Shuuchuu Dekinai

Yui, a high school senior, has a younger classmate who comes over to her house every weekend. Mari is a girl who — after a minor incident — became strangely attached to Yui and is always right by her side. Mari is funny, clever, and adored by Yui's whole family. But one day… Mari suddenly stops showing up. With college entrance exams looming, Yui tries to suppress her growing feelings and convince herself that it’s all for the best — but… A bittersweet story between a free-spirited kouhai and a studious senpai.

(Source: Compass Comics, translated)

Note: Locked for deletion because it was self-published under Compass and the mangaka does not yet have any commercial work.