My basic understanding is that you work for free, or a small stipend like food money or living arrangements, and gain knowledge from a professional. You might get lessons in exchange for your work. The work will depend on each trainer/rider. Some might just have you groom horses for them while others might have you muck out stalls and other barn chores. You can definitely be a working student in high school as long as your parents say yes and you find someone in your area that is willing to have you work around your school schedule.