Taiyou yori mo Mabushii Hoshi

Sae and Kouki have been friends since childhood. One fateful day at the end of middle school, she has a revelation: Kouki has grown up! Now they’re in high school, and she has another revelation: she has feelings for Kouki! But Sae isn’t the only girl who does… The question is, who does Kouki have feelings for?!
(Source: VIZ Media, edited)