I'm a sophomore in high school, but I'm graduating early next year
I'm going to Japan as an exchange student my "senior" year. Like you, I have really thought about being an English teacher in Japan.
I was thinking about becoming a business major (my mom really wants me to get an MBA) but I was thinking about just getting a BBA and then getting a job in Japan. The reason I want to get a BBA is so I have something to fall back on in case being an English teacher doesn't work out. Besides, you usually have to have a university degree, right?
Check out this channel-
meaphe's Channel - YouTube
She's actually in Japan right now as an English teacher, and she blogs about it everyday. Some of the stuff is kind of boring, but she seems really happy about teaching and living in Japan. I forget what program she's with, though.