My Youth Romantic Comedy Is Wrong, As I Expected @ comic, Vol. 5 (manga) — Wataru Watari

Spending summer break at home is the perfect scenario for loner Hachiman Hikigaya, but his club advisor, Hiratsuka-sensei, knows just how to ruin his dreams. Say good-bye to a quiet vacation and hello to taking care of some snot-nosed elementary school brats at summer camp! But there's a bullying problem among the kids, and the target is a pint-sized loner named Rumi Tsurumi! Can Hachiman and the rest of the Service Club help Rumi out, or is her social life done for?