If you want to teach high school you will go through the college (at the university) of the subject you want to teach. Math goes to the college of arts and sciences for instance. Then you will go through the college of education to get your certification for teaching. If you teach elementary you go totally through the college of education and probably get a generalist degree to teach anything they through at you for those younger grades. It easier to get certified higher and go lower than the opposite.

Go for secondary math or science because these are diffcult majors to get and few will attempt them so they are always looking for people to teach these subjects and you will have no trouble getting a job quickly at the school you chose. You may have to search for a school in different cities to teach anything else.