Does it bother anyone else that Draco spent his whole sixth year repairing the vanishing cabinet??? The deatheaters could have used a portkey to get into Hogwarts. Portkeys are underrated, there is no place in the book that says there’s a barrier around Hogwarts that prevents portkey travel. What about floo powder power? It would have been a year of sneaking around and a suspicious Potter avoided, excluding the trying to kill Dumbledore part.