Can’t recall the name of a supernatural manhwa.

Looking for a supernatural school life manhwa.

My friend read it not too long ago, and can’t recall its name anymore.

So the bottom is investigating the disappearance of a classmate and the blond top is a monster later revealed.
The chemistry and smut is really good but the finale end was good cause they went deep in the plot.
One of them had green eyes.
The bottom is actually a regressor but doens't remember.
Like there's letter and stuff written by him.
They had sex in the bathroom quite often.

It had the word “school” in the title at the time.