For reasons of security, the grounds and buildings of Hogwarts are protected by ancient Anti-Apparition and Anti-Disapparition spells, which prevent humans from Apparating on the school grounds. There is also a spell that prevents individuals from Disapparating, which Dumbledore places on the Death Eaters captured at the Ministry in "Order of the Phoenix"; a sister spell, which allows one to Apparate into a location but prevents them from Disapparating out of it, is cast by the Death Eaters in Hogsmeade in concordance with the Caterwauling Charm. Dumbledore is the only person who can both Apparate and Disapparate from the school grounds, since he is the headmaster.