Whilst everyone always seems to say "hey if you don't do well, it's all good cause you can go to UWS" - keep in mind that it is not the case that all unis require very high UAIs except UWS.
It is not an exclusive club that UWS belongs to

There are other unis that offer law degrees that don't tend to require 99+ UAIs - eg UNE (UAI cut offs have been in the 80s), University of Canberra, ANU, Griffith Uni, Macquarie (around the 95s from memory), Southern Cross Uni, Newcastle Uni, UTS (usually around the 97s which is a big difference from the 99s), Wollongong.
In reality, there are only 2 out of all unis offering law that are out of reach for students who don't achieve 99+ UAIs, which leaves a very broad range of choice for you.
Each uni has it's own advantages/disadvantages.
Yes, some may require a change of location, but if that is within your financial means, and if you are as intent on becomining a legal practitioner as you seem, it is worth considering.
As far as your concern as to the workload and marks required - you don't need to be achieving 1st place ranking in order to achieve a high enough UAI. I was certainly no where near 1st place ranking overall, probably somewhere around 40th out of 180 students at my high school, but I attained a 98+ UAI.
Although it does depend on your school and the subjects you do, it is definitely possible to gain a high enough UAI to enter law without being the top student. And apart from anything else - you still have a couple of years to go so there's every chance that you will improve between now and HSC time.
EDIT: Also, keep in mind, if finances are a concern for you - whilst you may have the marks to gain entry to Usyd, UNSW, UTS or one of the higher UAI entry courses, your chances of gaining a scholarship to a uni with a lower UAI entry are generally much higher. I really don't think attending a uni different to the one i chose would have made all that much difference to my learning and marks and employment options. Yes it may have benefited a little, but not so much to make the travel and lack of flexibility in timetabling etc worthwhile. BUT i definitely believe that receiving over $50k in scholarships over the duration of my course made it A LOT easier to do well which in turn made my choice of uni a much more beneficial one for me personally. If you are in a position of NEEDING to work to have money, gaining a scholarship can ease that pressure significantly and allow you more time to concentrate on uni. My scholarships main conditions were that I achieve above a credit average so in reality, its nothing major as you should really aim to do well just for the sake of doing well anyway