In Harry Potter and the Half-Blood Prince, Malfoy tries to get Death Eaters into Hogwarts through the Vanishing Cabinet. Couldn't the Death Eaters use an elf instead? We know they can teleport in and out of Hogwarts - Harry sends Kreacher to Hogwarts at the beginning of the book:
"Kreacher--I want you to go to Hogwarts and work in the kitchens there with the other house-elves."
Kreacher, who was now lying flat on his back with his arms and legs in the air, gave Harry one upside-down look of deepest loathing and, with another loud crack, vanished.
So why not save the effort and use a house-elf?