I am currently about to be a Junior in high school and I have been thinking about majoring in Music Composition. I enjoy to compose music and hopefully I will be able to enjoy that after college, but I’m worried about the lack of financial opportunities after college. Can I become a band director with a major in composition? What about with a minor in education? Should I double major in Math and Composition to have a bit more secure future? What jobs are available to composition majors? Can I join extracurriculars (e.g. marching band) with a double major in Math and Composition? Could I make a living composing custom marching shows for high schools? What about opening a business in drill design for high school/college marching bands? Could I also teach private lessons for my instrument (horn) on my own time? Thanks!
It is possible to support oneself solely as a composer. It is far from impossible, and there are many different ways composers can make money. As for composing for marching bands - it can be extremely lucrative. Check out John Mackey, for example. I recommend reading some of his blog entries: http://ostimusic.com/blog/ to get an idea of the life of a current successful band composer.
However, to be a band director, you’ll need to major in Music Education, I believe, and probably get a graduate degree in Music Ed, as well. Others would know more about that.
Thank you so much for this answer! I have read some of John Mackey’s blog and his blog is one of the best out there. I have always been interested in composing for marching bands, now I will look more into it. Thanks!
Music majors have access to the same jobs as any other bachelor’s degree holder, and the same access to grad and professional schools (Law, business, med, nursing etc.) w/proper prerequisites, which can be done after college if not done in college. I think it is fine to study what interests you on your college years, and go from there. You really cannot anticipate what will happen. (Some of us have kids who started composing relatively late but are now doing grad work.)
As your interests evolve, try and do some volunteering, internships or jobs related to what you might want to do for a career. But in the meantime, your studies do not actually have to relate to a career 
My main question with your anticipated plan is how much composing you have done, and whether you have performed works on recordings that can be used for a portfolio. You can certainly find BA programs where a portfolio is not required, but most BM programs do require 3-4 pieces, and your chances for admission to BA programs can be increased with an arts supplement with at least one recording.
Many music majors are classical (or jazz), and you would study theory, aural training, some piano for theory, ethnomusicology, composition and technology in music… Others will be able to tell you about specific programs that might be good for a musician/composer most interested in band music: I would guess state universities might be a good place to start your search with, but I don’t really know. Good luck!
Thank you for this answer! I will be thinking more about this.