A conical flask hurtles at the wall, hitting it with such incredible force a crack is revealed. The perpetrator storms out of the room, slamming the door behind him, leaving everyone - including Korosensei - in utter shock.

The perpetrator walks down the corridor, struggling to control his breathing. Some assassin he was... Couldn't even control his emotions. Now everyone knew.

Now everyone knew.

Nagisa can't take it anymore. The tension - the emotion - the silence, it's killing him. It's like his own personal hell. He can't save Korosensei. He can't fix his relationship with his own mother. He can't contribute well enough to class. He can't do anything, it seemed. But he knew that. He knew that from the day he was left alone.

All was silent in a park somewhere in Japan. One young boy, with blue hair and eyes, waited underneath a tree and sobbed openly, tears scarring his face, emotions scarring his heart. He had been there since three that afternoon. Now it was almost midnight. No one comforted him. No one ever would again.

That moment defined him. That moment made him who he was. That moment was everything to him, forever indented on his personality, on all his future relations, all his future prospects. Whatever he would do, it would all come back to that moment.

Korosensei's appearance is just a small part of his story after all. Gakushuu is the rest of it.