High Schoolers Playing in Japan

I don't know for sure, but am fairly certain that Japanese teams cannot draft non-Japanese players from foreign countries. There has been a case where a Brazillian-Japanese high schooler was drafted, but from Japanese a high school. If the high schooler in particular does not have any contractual obligations elsewhere, then he/she (can't be sexist) may be signed outside the draft as other foreign players are.

My advice is to read the FAQ (Frequently Asked Questions) about how to get a job with a Japanese team, and follow that advice. In particular, get an agent who's got experience with Japanese teams.

One thing that you'll want to keep in mind, though, is that NPB does not consider itself to be a prep-league for MLB. There are a great many unknowns about any foreign player, particularly whether or not (s)he'll adjust to living in Japan. Bringing someone so young over would be a high risk venture with little potential for immediate returns. While Japanese business tend to plan with long term goals, they also tend to shy away from high risk ventures. And ball clubs in particular don't plan on having their "migrant workers" (see Cromartie's "Slugging it out in Japan") around long enough to invest a few years of minor league ball to train them.

Repeat after me, "Nippon Professional Baseball is not a feeder system to MLB. Nippon Professional Baseball is not a MLB training league." Now I just wish I could convince the North American press of that.