I think by now there are enough works, especially YA works, set in some sort of variation on the real world that you can give it a genre name of its own, though I have no idea what to call it. But calling HP an epic fantasy seems logical enough, just like His Dark Materials, and unlike series like Twilight, or A Series of Unfortunate Events, which are less about saving the world and more about characters just trying to stay alive or live their life.