Archie is powerful because of the perfect match between his character and the situation. The situation is a closed community, with a history of power struggles and games, and of teacher/student and student/student clashes. It's a very charged microcosm. Into that world comes Archie, who is largely immoral, or at least a-moral, and willing to do just about anything to pursue his whims. He's willing to be very cruel, he's inventive, he's willing to blackmail, and he wants power. Combine that with his cunning, and you've got a dangerous mix.