From MangaHelpers:
Yotsuba Sachiko is the 25-year-old housewife of an 18-year-old high school boy, Yotsuba Tomoharu. She tries her best to get closer to him and do the things a housewife should do, but their relationship seems to be largely innocent from the beginning.

From Animexis:

On his way to school, Torii Akira helps a girl who was about to fall down. From that moment he keeps running into that girl, and she seems very happy to see Akira. Could this be the reason Akira can't help but smile all day?