Suki ni Nattara, Sayounara.

High school honors student Soya Furuhashi seems stern on the outside, but his underlying earnestness and naturally airheaded personality have made him well-liked by his peers. One day in the school bathroom, Soya meets a mysterious-looking guy who's drenched from head to toe. Wondering why he's never seen this student before, Soya later finds out that it was Motonari Kuga, the guy who was held back a year after rumors that he seduced other men...

(Source: Renta!)