The advice about a double major, or a fast track to an MFA is a great suggestion... I just want to offer another route to consider.
You can choose to study musical theatre in college, move to NYC, give it a go until you no longer want to, and then go to graduate school for education (or whatever... I have a friend who graduated with a BFA in MT, worked on Broadway as a performer for a few years, and is now a doctor [after many more years of school, of course
]) .
As far as public school teaching... Some states even have alternate routes to certification for people who have been out of school working in a profession other than teaching and now want to teach.
Many private schools do not require you to be certified. I recieved a BFA in MT, worked out of NY for a whileas a performer, decided that I no longer wanted to perform, ran the theatre program at the private high school for a few years before going to graduate school to get my MFA. I now teach in a college theatre program.
If you had asked me when I was 18 if I wanted to teach I would have said "NO WAY"... Now, when people ask me if I miss performing I say "NO WAY"... I like teaching and directing much, much more than I ever liked performing. But, I use all of my performance training, and professional experience as a performer in my work as a director and teacher.
Life is a journey. You do not come out of college "finished"... it's more like you come out ready to start... figure out what you think you want to do four years from now, and make a college choice that will help you get there. You may discover in college that you have passions and interests that you never knew you had that might open up the door to possible careers of which you never thought.
Good Luck!