Injoku Sousa ~Futari no Joshikousei wa Boku no Iinari~

Masato Nanjo is a friendless, timid guy who is constantly bullied by girls in his class. He learns certain secrets about those girls and decides now is the time for revenge. But he's not confident enough to take direct action, preferring to manipulate their downfall while hidden in the background.

[Edited from DLsite English]