This takes place in high school

You just left the classroom as you were the last person to leave, while you were walking at the corridor, you can't help but to notice a familiar classmate, it was Itsuha Sanzo, your classmate, who went up to the roof of the school, closing the door behind, due to curiosity, you walk up and slightly peek through the door, seeing her in an skimpy swimsuit leaning against the fence which caught you by surprise, which caused you to quickly close the door