From what my English teacher (head teacher) said, they actually calculate the number of words in an essay themselves.
What they do is they count the number of words in a line, and times that by the number of lines there are in the essay.
if the essay of importance, they will stop reading after so many words. But I guess it would be alright to go over a bit, say for a 1000 word essay maybe up to 100 extra words?
I think the word limit in a legal essay would be less important than one in an english essay so yehh
But then again, its better to be safe then sorry =]