Stringendo: Angel-tachi no Private Lesson

Stringendo: Angel-tachi no Private Lesson (2006) is a animation with 5 seasons created by Seto Yuuki, rated 7.2/10.

Tomohisa and Endou aren't either so handsome or ugly; they are just normal boy students. They're looking at Mizuho, the senior student, and Mikura, their classmate. Mizuho is a returnee student, and she attracts all the students by her beauty. Mizuho notices their eyes, and she gives Tomohisa a suggestive look. One day, Tomohisa finds a note in his desk. It says, "I'm waiting in the ladies' room on the third floor". He goes there and finds Mizuho is waiting for him... The next day, Tomohisa can't help recalling the event the day before. He happens to find that Endou is gone, and also Mikura, whom he loves secretly, is missing, too. He's afraid that they're doing things. However, exactly, they're doing things on the fire escape.