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Once you get to high school, I would highly suggest taking Anatomy & Physiology (this was an optional class at my high school). This class will be a great introduction into a lot of what you'd be learning in a Kinesiology or Exercise Science major which are great degrees to pursue to become a personal trainer. Middle school and high school classes are really there to help you "learn how to learn" and it's when you get to college where you'll start to really specialize your classes to your future career. While you don't need to get a college degree to become a personal trainer (you can substitute a degree with a certification course), it will certainly help by giving you more credibility and providing you opportunities to network. Networking can lead to some great, real-life experiences/internships which I agree with Zach, will be very important and will help you stand out once you start your career as a personal trainer. Additionally, a lot of intercollegiate athletic programs offer student internships throughout the year so you'll get the opportunity to shadow both the medical trainers and the weights coaches and work with actual student-athletes. I was a collegiate springboard diver and worked a lot with the student trainers who've said that their experience has been incredibly useful after college.
In high school, you will have to graduate and that will require all kinds of courses but when you look into college it will be a lot more sciences. Focus on science and do a lot of self-study to start. Look for opportunities to job shadow trainers and even get a part-time job at a fitness club. The job market is very competitive so experiences will be very key to getting a good high-paying career position in this field.