After many in-depth studies and intense research on the subject, I still have not been able to glean any information on the casualty rate of the battle. It's been bugging me for some time now. I've looked into every history I can get, and still haven't found much accurate information.
I would also like to contest that a flanking move by the Death Eaters in the moments before a standstill was called could have crushed the Order and their allies, leading to a total victory for the Dark side. What does everyone think?
Whilst an important historical event in World and Wizarding history, I believe this may fall foul of the /r/askhistorians/ 20 year rule :(
The fact of the matter is that, while it is an important moment in wizarding history, calling the battle of Hogwarts a battle is a bit of an over statement. The school only housed a few hundred students at the time, of which 200 at most were of any use in combat. The Death Eaters, totaling just under 100, should have won in a dominating fashion just by their superior abilities had it not been for their leader.
Tom Riddle (or as I'm sure everyone knows he liked to be called, Lord Voldemort), had been obsessed with a young man named Harold Potter for some time. Indeed, somehow Mr. Potter had survived an attack by the leader of the Death Eaters when he was just a baby. Now there is some rumors of a prophecy of some sort dealing with these two that was held within the Ministry of Magic, but no record of such a thing exists and all instances of prophecy throughout history seems to be something of a stretch. So, for whatever reason, Voldemort felt that if he were able to kill Mr. Potter that the battle would end. Luckily for those defending Hogwarts, Potter was able to feign death, and Voldemort returned to the school for what he thought would be a quick surrender.
This was ultimately the Death Eaters undoing. Voldemort had brought all of the Death Eater leadership to the Great Hall of the school, and when Potter revealed his feint, the Defenders quickly surrounded the head contingent of Death Eaters. While those inside the hall fought, the students and faculty outside the Hall were able to use the main gate at a choke point to prevent any reinforcements from joining the main melee. After approximately an hour and a half of fighting, the heads of Death Eaters had all been defeated, including Voldemort, who had been killed by Mr. Potter himself.
All in all, not only do many accounts of the Battle of Hogwarts put too much emphasis on the actions of Harry Potter (not to say they are not important), but really the battle was the Dark Lord's to lose, and he managed to do just that.
You see, amateurs talk about tactics while professionals focus on the logistics. A flanking move would have been impossible as Hogwarts' floo network was down due to a curse, the knight bus was in the 14th week of a strike over benefits, and a recall of all brooms after 2001 due to shoddy parts (imitation salamander hearts).
Furthermore attempting to cover that terrain on foot would have taken the Death Eaters through a muddy field just after a rainstorm. The entire flanking force would have been bogged down in the mud. While a garden path is mentioned in some accounts, Rowling (2007) is explicit when she says that was only conjectural and wouldn't be completed until 2010 when the headmaster grew tired of ruining his nice new shoes.
Finally there's the food element. To undertake a flank march the Death Eaters would need to be fed but no one had forseen to pack lunches. Voldemort had only planned a crafts table serving soup in flimsy containers due to financial problems (the galleon market had tanked and the dark lord had over-leveraged himself in futures). Clearly the Death Eaters couldn't carry soup in cheap styrofoam bowls the 0.78km much less struggle through the mud without spilling the nutritious brew.