It Starts With A Letter

Natsuki's school life couldn't be any more boring. Taking notes, listening to teachers, someone threatening to expose her darkest secrets through a letter placed in her locker-

"Is this some sick joke? Nobody from this school even talks to me to know about my secret!"

Natsuki, now forced by her blackmailers demands, joins The Literature Club. Just who wrote the letter? And why did they make her join this club?