Debate/Public Speaking, Business (if required to present or perform sales presentations in class) and any class that would force you to dig into things -- anything with a hypothesis and/or research paper where you must defend a perspective or point of view.
There are no perfect course selection for law as it fits anything where there can be disagreement -- engineering, science, business, politics, health, you name it. Instead think of law as an aspect of an area. You can be general lawyer, trial lawyer or specialized such as business, patent, etc.. If there is something you love, focus on that being sure to find every opportunity to speak in front of groups and persuade them using research, logic and sales skills.
In my opinion there is not one class or subject that will help you succeed in a career as a lawyer. It is important to remember that "law" is a very wide and varied field of work. You have some lawyers, like myself, who have practiced corporate law, while others have acted as criminal prosecutors, human rights lawyers or even tax or real estate lawyers (just to name a few). The key to being successful as a lawyer, irrespective of your particular specialization, is the ability to think circumspectly about a particular problem and analyze the fact pattern to devise an appropriate solution within the framework that the law creates. Therefore critical thinking is incredibly important. Courses such as math, logic, history, english, the sciences, civics - these all contribute to your ability to think critically so I think that you should strive to take as many courses that interest you without worrying too much about whether they will lead to a career as a lawyer. Some of the best lawyers I know have worked in totally different professions before becoming lawyers. I also think that the key to being successful in whatever career you choose is to do something that interests you and that you are passionate about. As a high school student I would also encourage you to study a foreign language (or two) and be sure that you take any opportunity to travel the world and experience different cultures and ways of life because all of these experiences will contribute to broadening your knowledge of the world and of people which in turn will help you to be a better lawyer if you so choose. Remember that the importance of the law is its application to real life situations so the broader your life experience and knowledge of the world the better a lawyer you can be. I hope this helps and I wish you the best of luck!
From my experience as a student and a lawyer, I wouldn't recommend planning your high school cirriculum to prepare for a career in law, for a couple reasons.
First, if you're starting out and trying to plan your high school course strategy, you are probably 6 to 8 years away from law school, and that is too far out. As you continue to learn and grow, your interests and passions are likely to change. My advice would be to keep an open mind and open to change directions if that is what you are called to do in your life.
Second, I really don't think there are specific high school (even college) courses I would recommend as essential or critical for an inspiring law student. My advice would be to take a broad range of courses that allow you to get comfortable with certain foundational subjects, like language arts, reading comprehension, logic, and other critical thinking skills. Most high school curricula and advanced curricula are designed to cover the development of these critical thinking skills.