YURI NO HAJIMARI WA DOREI KARA

Chiyoko Hanamura is a girl who has failed in her attempt to remake herself into a “gal” for her high school debut. While skipping class on the rooftop as usual, a kouhai girl, Sakine, comes to ask her for a favor, showing her an anonymous love letter. Sakine asks her, “Can you go turn down this confession for me?” For the sake of her cute kouhai and to maintain her dignity as a gal, Chiyoko accepts the request and sets out to decline the confession, not knowing who would be there… (Source: ComicWalker, translated) — Description from AniList community (anilist.co).