Jijou wo Shiranai Tenkousei ga Guigui Kuru.

Takada-kun, the new kid at school, has no idea that everyone in the class calls his classmate, Nishimura-san, the "Grim Reaper." His only thoughts are about what a cool nickname it is. Nishimura-san is relentlessly teased while her classmates snicker behind her back, and Takada-kun is oblivious to the whole situation. Nothing beats being clueless and honest, and that's why his feelings reach right to her heart. This story that starts in an elementary school classroom will make anyone remember those days by reaching right to their hearts.

(Source: Crunchyroll)