The First Rule at Hogwarts wasn't something like "No magic in the hallways." as one would expect. Instead, written and later printed in large words that had been underlined several times was "If a man with a blue box offers you a ride around the universe and promises to have you back on the Hogwarts lawn before anyone realizes you've left, DO NOT TAKE HIM UP ON HIS OFFER."
Sally-Anne Perks, like God only knows how many students before and after her, had found herself compelled to break this rule. After a rather amazing adventure in which some of the statuary surrounding the castle had been destroyed, the man in the blue box had pointed out that his friend Adric needed friends his own age to hang around with, and offered her all of space and time, promising to have her home by dinner. After a journey that had lasted at least a month, The Doctor dropped her off on a wide sloping lawn as the sun was setting, said goodbye, and vanished as if he were never there.
Smiling mischievously at the thought that she'd managed to pull one over on the teachers and play hooky for a month, she turned to make her way into the castle. The thing was, what she saw when she turned to go inside was not the familiar medieval castle she was familiar with, but a massive citadel that looked as if it could house hundreds of thousands of inhabitants instead. Instead of moving forward, she found herself frozen to the spot staring at the shining silver building as one of its occupants made their way outside and towards her.
"Sally-Anne Perks?" the witch in the strange robes asked upon reaching her.
"Y-Yes?" she squeaked.
"Welcome to New Hogwarts." the kindly older looking witch said. "Well, technically it's New New New New New New New New New New New New New Hogwarts, but you get the idea."
Meanwhile in 1472:
The Headmaster sighed as he headed out onto the lawn. Hogwarts would still be sentient if it weren't for these occurrences. These occurrences which drained magic from the castle at a prodigious rate, almost taking it faster than the building could absorb it from the ley lines upon which it had been built.
"Follow me." he said when he reached the students who apparently didn't recognize him if their reaction was any indication.
The students followed as he made his way into the castle and up towards the Room of Requirement.
"Where are you taking us?" one of the students asked.
"To the hibernation chamber." he replied.
"The what?" asked another student who apparently hadn't been born after the Muggles' so called Industrial Revolution which had introduced any number of strange ideas into Wizarding society such as the hibernation chamber that the Founders had been forced to create.
He said nothing more, choosing instead to pace before the entry and think on what he wanted. Moments later the door to the room where hundreds upon hundreds of students lay in stasis awaiting the arrival of their own time appeared. Upon entering, he guided the group of students to a set of vacant shelves which had been created according to his need.
As he got the brats situated, he noted with satisfaction that Michaelson was due to wake up soon, and when he did, the boy was going to be getting a thrashing he'd never forget.