If you take away the whole ghost thing, The Turn of the Screw really becomes a story about social class. Interestingly, it's still a horror story. James uses class difference to create much of the tension in the story; sure, it's scary that ghosts might be menacing a couple of adorable children, but what's even scarier, he tells us, is the idea the one of these ghosts might be a common, working class guy, who's endangering a rich, upper-class boy just by association.
The true danger posed by Peter Quint in The Turn of the Screw is not a sexual threat, but one of class, since he threatens the boundaries between servant and master.
One of the most important qualities valued in The Turn of the Screw is knowing one's place in the world; however, our protagonist, the Governess, consistently steps out of hers, demonstrating that she is just as much a transgressor as either Quint or Jessel.