Nagai's ecchi parody of Moonlight Mask (Gekkou Kamen).

Welcome to the Sparta Acadamy, run by a head master called Satan's Toenail. If you get anything below a 90 on every test, they punish you with torturous devices and the like. But fear not, there is one who stands up to this great injustice, in nothing but a mask and high boots. That's right, a naked fighter of Justice, Kekkou Kamen is here to punish the unfair system of the Sparta Acadamy and bring justice to those who study too much!