Tsuzura-senpai wa Motokano dakara

Ritsu Osanai is a high school boy who wishes for nothing more than a perfect, peaceful life. But at the high school he attends is Sana Tsuzura, his senpai and the "ex-girlfriend" he dated back in middle school...!? His high school life, constantly thrown into disarray by his unpredictable ex-girlfriend, is about to begin...!

(Source: Shueisha, translated)