Let's start from the beginning, shall we?
It was an extremely regular day at Hogwarts. The sun was shining; the hippogriffs were flying happily in the meadows. Harry Potter then awoke to the melodious sound of his best friend Ron Weasley inhaling a pillow. Harry oozed out of bed and looked in the mirror, to discover that his hair looked as if it had been mauled by a manticore.
After much preparation, Harry finally got to class, preparing two hundred pounds of books and papers. As he clambered up the stairs he said hello to Hermione Granger, his other best friend, who was carrying five times as many books as he was. Not interested in having a conversation at one in the morning, Harry slumped into his desk. Harry then reflected on the previous day and screamed.
Yesterday everything had turned upside-down...literally. It all started when Voldemort hit all of Hogwarts with a terrible spell, creating a giant vortex of absolutely everything. When Harry had woken up that morning, the first thing he noticed was that Ron was bald, and his bed had mysteriously been nailed to the ceiling. The drapes were discoloured, and absolutely hideous. And, of course, Voldemort was behind the whole thing with a terrible scheme of evil screwing-up-of-everything. So now, everything was completely out of order, unorganized, and just outright silly.
And now, for no apparent reason, all Defence Against the Dark Arts classes had been re-scheduled to one in the morning, something which greatly irritated Harry. As he entered the classroom, Harry noticed that there was no-one there, and the teacher was nowhere to be seen. Then he heard a strange noise coming from inside the closet. Harry walked toward it, curious of what could be causing this strange sound. As he swung open the door, he found Snape bound and gagged, stuffed inside the closet. Harry was not surprised whatsoever.
As he closed the door to let Snape be miserable in private, he turned around and headed toward his desk. As he seated himself, only he, Hermione, and Ron were left in the classroom. Apparently, no-one else had bothered to get up. Then all of a sudden, in a puff of orange smoke, a strange, thin man appeared out of absolutely nowhere. A monstrous grin split his face as he leaped onto the desk and cried, "Good morning!", then proceeding to crow like a rooster.
While these shenanigans were in play, Ron attempted to sneak up on him with a straitjacket. The attempt failed miserably as the strange man disappeared, shortly thereafter reappearing within a drawer in his desk. Don't ask me how he got there, I have no clue.
The strange man sprung in the air, and after unfolding himself from his origami form and gathering his composure, cleared his throat and formally introduced himself.
"Hello, class...of three...? I am Professor Cedric, your new Defence Against the Dark Arts teacher."
Harry stared at him in disbelief. Ron drooled on the desk with minimal participation. Hermione shrieked in terror. Professor Cedric continued.
"Now, let us begin". Professor Cedric then removed a large white sheet from what appeared to be a painting consisting of dark colours. One Avada Kedavra later and the painting was reduced to canvas confetti. Professor Cedric then called Ron up. "Now, you try, Mr….uh….umm…Ferret!"
"It's WEASLEY!" Ron shrieked. Professor Cedric then revealed another dark painting. Harry mumbled to himself, "This is completely and utterly bogus!" Then, Hermione leapt from her desk, flew across the room, snatched Ron's wand out of his hand, and snapped it cleanly in two. She then proceeded to shove the broken pieces of the wand up professor Cedric's nostrils.
"Well, what did you do that for?!" professor Cedric said nasally.
"Don't you know the Avada Kedavra is illegal, professor?" Hermione yelled.
"It is? Since when? I use this spell all the time to cut my vegetables for dinner!" professor Cedric replied. And so, the class, that is to say Harry, Ron, and Hermione, charged out of the classroom like greased lightning, falling down six hundred flights of stairs.
And this was how it came to be that Harry Potter was sitting in a desk in an empty classroom, wincing in pain and screaming his bloody head off.