Evil High takes place in an alternate present, where super heroes and villains have been feuding for decades. Evil is not seen as a blight that must be wiped out, but as a necessary counterpart to Good. Villains are accepted as a part of society alongside heroes, and though most parents would rather their children become upstanding citizens, many tell their teens to follow their hearts, and become whoever they really want to be.
This is where Evil High comes in. When the Hero is strong, the Villain must be clever. Shrewdly aware of the world, and keenly observant of the weakness of his foes, the villain uses his mind, not his muscles, to overcome the strength of his enemy. Evil recognizes and appreciates a sharp mind above all things. This is in part, why Evil High is recognized as having one of the most demanding curricula of any school in America.
Evil High's classes are so demanding, that the junior year finals are believed to be literally impossible (leading to the myth that you only become a Senior by being intelligent enough to cheat, and not get caught).