[Hatredcopter]

The chances of a foreign person teaching anything but English in a Japanese school are pretty dismal. Ask yourself why a Japanese school would need someone whose native tongue is not Japanese and hasn't spent their whole life growing up and living in Japan. Even if you're more qualified for a position than any potential Japanese candidate, they'll choose the Japanese candidate, just because he/she is Japanese. Sound unfair or discriminatory? Maybe so, but that's how things work over there.

To start with, you'd have to be completely fluent in Japanese. You'd have to be educated and licensed as a teacher by the Japanese education system. You'd likely not be making much money, and you'd probably end up being pigeonholed by the school into doing English-related projects anyway.

I don't mean to paint such a dim view of your dream job, but that's the reality of things in Japan. Becoming an English teacher would be infinitely easier than becoming a regular schoolteacher in Japan, and in many cases, you'd probably still make more money than a regular teacher.

Higher education, however, is a different story. There are a number of foreign instructors working in colleges/universities in Japan, and many of them don't teach English. Of course, for this, you'd have to hold (at the very minimum) a Master's degree for an entry level position, on top of already being completely fluent in Japanese.