To thrive in legal education, you will need:
-excellent comprehension and written expression
-strong analytical reasoning
-work ethic
The first two will be well-developed in the humanities. English is often trotted out as an indicator of suitability for law at uni, but I would suggest that something like Modern History/History Extension is much more relevant. In those subjects and others like them, you are drilled in how to present a well-expressed and coherent argument. Conciseness and precision is preferred above literary flair, and solid research is a must. You are expected to read widely and filter out the relevant information, and to critically examine the reasoning of the historians you are reading. This sort of work is the bread and butter of uni law.
Legal studies is a good subject to do if you have a genuine interest in what that course involves. Don't do it expecting it to give you an edge in law school--it won't.
Extra-curriculars like debating will help you develop your reasoning and quick thinking. Whether you want to fulfill every aspect of the law student stereotype is your choice!
The third is something that depends on your own character, choices and experience. Some people quite reasonably opt to focus on leadership/work/co-curricular stuff at uni, and sacrifice grades accordingly. Others power through on Ds/HDs consistently. A brilliant few can do both at the one time. Ambition will help you in law school, and also in your HSC. I know when I was in high school, keeping my mind on my tertiary goals helped me survive the drudgery of the final years of high school. Unfortunately, uni is just as stressful, and though you might love the law, you are guaranteed to absolutely despise some courses, and it will really suck at times. Work ethic, I think, is what really tends to divide the sheep from the goats at uni. Everyone who gets into a law course obviously has half a brain and has demonstrated a certain level of competence. The academic powerhouses you meet at uni are sometimes geniuses, but more frequently just absolute library rats.
Having said that, there is no way for your HSC subject choices to define how well you will do in law. Uni and school are two entirely different undertakings. What is most important, I would wager, is your ambition, your work ethic, and your intelligence. If I were you, I would focus on the HSC and let law school take care of itself. Do the HSC subjects which interest you most, and which you think will allow you to fully demonstrate your ability and gain an optimal UAI. As you walk through the gates of uni in O Week, your slate will be wiped clean. Your UAI, HSC experiences, the school you went to are all largely irrelevant. You will realise that uni, and law school particularly, is a new game.
Also remember that law is a field which, for all its grade-whoring, is also very much about who you know and what experience you have. Follow up any connections you might have. Do your work experience in a law firm, or legal aid. Volunteer time doing shitkicker work at a local community legal centre, if such a thing is available. Try and score a part-time gig at a law firm in your gap year, etc. etc. It's a good idea to demonstrate interest in the law and also get a close-up look at the work involved, to see if it truly is the career for you.
Good luck with your HSC.