Seieki Tairyou Chuunyuu! ~Ore no Kodane wa Biyaku Iri~

The day after the protagonist was bullied by a dominant classmate, he finds that suddenly his testicles and penis have grown to enormous proportions!

He rushes to the the infirmary, where he is told they will need a sample of semen. However, he ends up ejaculating an unbelievable amount.
When the nurse sees this, she tells him:
"Why don't you take revenge on the girls who bullied you with your new toy?"

[From DLsite English]