Summary: War has been raging in the wizarding world and Hogwarts has been attacked! This is what I think happens during the attack.
Attack on Hogwarts
The sun could be seen shining on the lake and everywhere else illuminating the darkness and the signal for a new day. It seemed like another normal day, no one would have thought that this day something major was about to happen.
"We have been attacked" yells a first year as the doors to Hogwarts are blasted open. Swooping in are people robed in black with masks on their faces to hide their identity. This is odd because they walk around as though they are better than everyone yet they have to hide faces like cowards. Their wands out in front of them ready to attack.
The students start panicking and running around trying to get away from them. The prefects and head boy and girl yell fruitlessly at them to follow them to safety, but it is in vain for all the students run for their lives not looking back to see what happens to the others.
They run into their dormitory and common rooms thinking no one will get them over there and some run into classrooms to hide under desks and still others just stand there and begin to cry in fear until their fellow classmates come to them and drag them away.
Yet there are some students who stay back to help protect their beloved school. They stand ready for duel with their wand held high and trying to fight the fear that was demanding to subjugate them, instead thinking about all the hexes and spells they had learned to use on the enemy that had just entered.
Dumbledore stands in front of everyone with an expression of cold fury on his face, the twinkle that was always there in his eyes lost, seeming as though it would never return. Behind him stand his faithful teachers, all ready for battle.
The black robed figures stop as they realize that Dumbledore is there, there must be some misunderstanding, Dumbledore is not supposed to be here, he is supposed to be at the Ministry. Lucius Malfoy himself had sent an owl to Dumbledore addressed from the Minister so he wouldn't be here to defend Hogwarts and it would be easier on them to attack the school.
Oh well now that they had attacked there was no turning back or else their Lord would be most displeased with them. They shuddered at the thought of that, no one wanted to displease him otherwise he would punish them and his punishments weren't something that was easy to forget.
All is still as though both sides are waiting for the other to make the first move, like it was planned a black robe figure from the middle steps forward and points his wand at a second year and yells an Unforgivable curse only to be deflected by Dumbledore.
This catalyst begins the battle for the school.
Curses start to fly from opposite sides of the room, most missing their desired opponent, yet some hitting what they intended on hitting. Chaos has erupted once again in the school and the children remember the panic yet again and try to get out of the way. Most of the students are gone into hiding in the school but some aren't lucky; a first year is hit with the death curse and crumples to the ground with a look of regret on their face of why they had ever stepped foot in this place.
The ghosts of Hogwarts start to come through the walls with a look of pure venom hatred etched onto their faces followed by Peeves who for once isn't glad at the sight of chaos as he generally is.
The teachers take on two or three black figures at a time while Dumbledore takes on about a half of them, who are all cowering under their mask at the sight of him as he captures one by one. The brave students who stayed behind to help defend their school aren't having that much luck, they have to stay on the defense because their enemy wont let them attack, being to swift for them, yet they still don't give up and keep fighting.
The ghost chase some of the black figures out of the castle making them shriek in pain as they take turns passing through their bodies, staying inside of them and screaming as loud as they can in the enemy's ears. While Peeves pulls mats under them and dump walking sticks on them and then picks up a walking stick and start beating them until they are unconscious.
It seems as though the school is putting up a good fight.
The school shudders as it once had four years ago when it had to baby sit those wretched creatures. Every where it starts to get colder, the plants in the Forbidden Forest begin to wilt and everyone starts to live their worst memory.
Shouting begins to erupt as they get closer to the school; shouting of agony, sadness, and guilt. Sadness can even be seen, though very slightly, on Dumbledore's face. The creatures sense this and are pleased that there are so many victims to feed on. Their master had not lied to them, promising them all the humans they wanted. And now they were going to take them all they thought hungrily as they flew into the school.
A small third year is in the corner crying as he thinks about the day his brother died; it had been the saddest day of his life. As a creature passes by it sense this and swiftly goes over to the boy, the boy screams in fear and then he becomes quiet all of a sudden, a blank expression on his ghostly white face, he blinks his eyes stupidly and just sits there never to be the same again as the creature leaves him and looks for another victim to suck their soul out just as it had done to the boy.
Hagrid comes with a crossbow in his hand, behind all the monsters in the Forbidden Forest that he could persuade to come. Though Aragog couldn't come because he could barely move for the reason that of old age, he sent his family to help, and they don't hesitate for minute as they start to climb up the foe biting everywhere and making them bellow in pain and discomfort. His other friends started charging at the foes, some get hurt and others die. The centaurs who had made up with Hagrid and Dumbledore as well, start attacking the Dementors because their powers have no effect on the centaurs chasing them out of the school.
There is fighting everywhere, in the library, in the Great Hall, in classrooms, outside of school, even in the bathroom where Moaning Mrtyle is at. The robed figures even try to get inside the common rooms but the guardian of the common rooms don't let them in, putting up a good fight to protect the students who are inside hiding in fear and hoping and that they don't get through to them.
It seems as though the enemy are going to be victorious, but what they don't know is that the school holds magic that had once resided in the Four Founders of the school. When they had built the school, they had agreed that once they died there magic would go into the school to be used if ever the school was under siege.
And use the magic the school did, with a last final attack from the school, a sword could be seen wielded as though by an invisible person as they started to assault the enemy, following a large venomous serpent that started biting the black figures, a bird flew in from within the school and started to peck the heads of the enemy and pulling their mask off to try and scratch their face off, with a badger following the others trying to cause as much hurt as possible; no attack of any kind affected them for they were protected by the magic of the Founders.
Realizing that it was all over, the few of the enemy that was left ran as fast as they could to the front of the school, so they can apparate from the school before they too were caught.
The aftermath isn't nice, bodies lay all over the school, some dead while others unconscious waiting to be tended to. Most of the bodies that covered the ground were of the students of Hogwarts, the innocent that were there at the wrong place at the wrong time, they had to pay for it even though they had nothing to do with it.
There was destruction everywhere. Half the library was burned down, and almost everything was smashed and destroyed except in the common rooms, if it wasn't for the portraits that guarded those rooms, they would have been destroyed as well and more deaths might have occurred.
Sadly the remaining people of Hogwarts start to fix what was ruin and get the injured to the infirmary as quickly as possible.
Students mourn as they pick up there friends from the heap of rubbish that is strewn everywhere, hoping that they are alive as they drag them up to the infirmary for Madame Pompfry to take care of them.
The safest place on earth was attacked, which means that they can do it again, so now the school has to stay alert for the next battle for the school to occur.