Flowers of Evil

Takao worships the class beauty, Nanako, from afar. When he is alone in the classroom one day after school, he notices her bag of gym clothes on the floor. He can't resist picking it up. He plans to return it, but the next day, all the girls are commiserating with Nanako that some pervert has stolen her gym clothes. Worse than that, their classmate Sawa quietly lets him know that she saw him take the clothes and will tell Nanako if he doesn't make a contract with her to do whatever she asks of him. And so their strange, tense relationship begins.

Synopsis from Anime News Network