Naburi Yuugi

The protagonist, Riku, is a plain, uninteresting student. He's never had a girlfriend, only devotes himself to his job, and even in school he doesn't have a lot of friends.

One day, he recieves news of a meetup of alumni from his old middle school. Riku remembers the bad memories he had of middle school, being bullied by three of his classmates. But, bored with his unchanging daily life, Riku decides to go to the reunion anyway.

Without realizing it, he opened a world for himself full of despair and violation...

[slight edit of Getchu synopsis]