Harry Potter's world isn't real at all, and I don't believe the author's purpose in writing was to make the reader believe it's real........ it's all about imagination. As far a the plot, she does add age appropriate problems and teenage/ young adult angst when she can. We've seen family problems and crushes, competition between peers, loyalty, and a number of other things questioned. To me, those are the only things that happen in the "real" world.
As for as the Slytherin House, what would they do without it. Its existence provides us with a number of conflicts and plot twists, and it's inhabitants are some of the best antogonists to be found. We need the Slytherin House its endless possibilities.