Ore no Ichizu wo Jama suru Aitsu

Wakaba Osanai, a fifth-grade boy, is in love with his homeroom teacher! To become a man worthy of his teacher, he pretends to be more mature than he really is! But his childhood friend, Kai Wakanouchi, begins teasing him after he reveals his feelings for their teacher. Kai has feelings for him too... and she's trying to catch is attention by flirting!

Thus begins a "double unrequited love" romcom, featuring a boy struggling against the seductive antics of his childhood friend!

(Source: Ichijinsha, translated)