In the books, there was an all-out attack on Hogwarts when the Death Eaters broke into Hogwarts. Fenrir Greyback runs around saying, “I…like children” and so forth. It’s quite creepy and kill.
They cut this from the movie because it felt like it would simply be repeated in Deathly Hallows, when the final battle at Hogwarts really will be shown. I think it was a combination of David Yates’(director) and Steve Kloves’(writer) decision.
Part 2 of the Deathly Hallows is rumored(though not confirmed) to have a good 15-20 minute final battle at Hogwarts. With the drama of it, it should be pretty cool.
Also, we are told (though I don’t recall in which book) that Hogwarts was the only place that Voldemort ever felt was “home.” It makes sense that he would want to seize it for a base of operations.
i believe that was book 6 as well. part of the lessons of dumbledore.
although, one could argue for book 2, as riddle set up hagrid for the opening of the chamber of secrets. after myrtle’s death they were going to close the school and riddle had asked permission to stay the summer. he framed hagrid and the school stayed open.
his memory stated in the chamber that he didn’t dare open the chamber again while he was there.
The earliest mention comes from Dumbledore when he mentions he got lost in the castle one night while needing to use the bathroom and runs across a room full of antique chamber pots.