Well, to add to what has been said, we know that Voldemort
made it into Hogwarts in HBP, or his cronies did, at least. He did it once, he can probably do it again. With Dumbledore
gone, I doubt he would fear anyone else there enough to keep him away. I can't imagine why he would want to go there though unless it's to get an artifact or a hidden Horcrux. Maybe he thinks he can illicit terror into all the witches and wizards by showing them that their children are not safe even in Hogwarts. Maybe Voldemort
will take over the school for no other reason than to draw Harry
there because he knows Harry
would go there to protect his friends.