One course you could look into studying while still in High School is "Anatomy & Physiology". It's pretty much one of the only course we took during our studies of massage that required the largest amount of book work, and it seemed to freak a lot of students out but only at first.
Also look into doing some things personally on your own, like practicing Yoga, Meditation Classes, Movement Classes or Thai Chi, all of which will help you to become more self aware and put you in an environment of individual who are Massage Therapist themselves or curiously like minded about the Healing Arts.
After Graduating from High School you could go to a college and study the 'Art of Massage' but you would be better off studying & receiving your Certificate of Massage at a School of Massage.
Visit one of your local Schools of Massage, they should be able to answer any & every question you may have about other things you could possibly look into studying on your own as well as course you could take while still in High School that could prepare you for the healing environment that you'll find yourself in when you attend a School to become a Massage Therapist, as well as help you decide on a Modality of Massage that suits your personality best.
When many people think of a massage they only think of Swedish Massage but there are countless other modalities of massage, some of which being Shiatsu, Reflexology and Thai Massage : All of which are become very popular as the public becoming more & more educated about them.