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Your options are pretty limited if you haven't finished secondary school. You can't finish it up in Japan. You might be able to attend a language school in Japan, but after you're done with that, you have to leave Japan. There are no other schools below a university-level school that you can attend, as they all require you to be completely fluent in Japanese - this includes both high schools and university prep schools (cram schools).

If you really want to live in Japan, you'll have to finish up school. Visa requirements are strict there. You can't even get a job without having a bachelor's degree from a four-year university.

Bottom line is... not really possible, sorry to say. Like I said, it might be possible for you to attend a language school for a few months, but I can't say for sure.