Most music programs will require you to study vocal music, so even if you want to teach instruments rather than voice, learning to sing can be beneficial. In addition, taking a choir class will help you to improve your ear for pitch and tone quality, learn how to blend with an ensemble and know how to sit, stand and breathe for good air support. No matter what kind of music you want to teach, these lessons will serve you well.
If you do want to teach vocal music, learning at least one foreign language can be invaluable. An enormous amount of classical and operatic music is in foreign languages and being able to pronounce and understand the words in these songs will be extremely useful. In general, your best choices include Italian, German, Latin and French. Even if you don't want to sing, studying Italian can be useful because so much musical terminology is in that language.
Whether you want to teach vocal music or instrumental, taking a band class is essential. Music colleges almost always require knowledge of piano, at a minimum. If you want to be certified to teach band, you will need to develop basic familiarity with every common band instrument. The sooner you start learning them, the better. If you only need to learn piano for your concentration, try playing mallets - they are a common band instrument with the same basic layout as a piano.
Other humanities courses are also extremely valuable, including English, history, and art. English will help you learn how to analyze writing, which will come in handy when you must examine the words you are singing. Music history is often required by college programs, so developing a basic awareness of history in high school will make things easier for you later. Studying other forms of art can also help improve your ability to express yourself in all artistic media.