Hogwarts is not what it used to be!
Notice how much the geography of Hogwarts changed between films two and three. Could be because..
There will be a war. Hogwarts will be attacked. Giants will be involved. Hogwarts will be easier to defend. It will look better on film if Hogwarst is on a hill.
Also in this was I am expecting all four houses to join together against the threat. Some in Slytherin will have split loyalties. My guess is Malfoy will chicken out or reject Voldemort. He does'nt match up to his fathers bad ways - more like his mother.