If the word limit is stated as 500, it really means there is no word limit, and you should try to write as much as you can (your chance of getting into the school increases by 0.1% for each extra character you go over the limit).
Seriously, though, why would you try to stretch such a definite boundary? I think most schools put in a built-in limit into the online secondary that prevents you from submitting an essay that goes over the limit.