Incidentally, Hitomi and I watched a South Korean documentary on a (North) Korean school run in Hokkaido, Japan called "Our School". (The trailer is below and unfortunately subtitled in Korean not English. Click on the link for a summary.) The gist is Koreans living in Japan feel like outsiders and are more comfortable in an environment where they can speak Korean and not be treated like a minority.
Being Korean in Japan is tough, because Japanese-North Korean relations aren't too great: The occasional Nodong missile is fired over Japan, people are still upset over the abduction of Japanese for spying, North Korean ships are notorious for smuggling in drugs and receiving cash from Japan, a backlash against the recent Korean Wave. Hitomi says Korean kids are aggressive, which is off putting for many Japanese.
You can get the documentary from Scarecrow Video in Seattle or probably through YesAsia and places like that.
Not mentioned by the documentary is the curriculum, which Hitomi pointed out did not follow the standard Japanese curriculum, thus Korean high school was treated as a trade school and students were not eligible for the University without taking a test. I suspect that given the North Koreans were subsidizing the school that at least the history lessons would have a bit of pro-Communist, pro-North Korean bias.
Knowing Japanese, it's interesting listening to the students and even adult Koreans speak, since they mix in Japanese and Korean all the time, in the middle of sentences. The students themselves make a special effort to speak "100% Korean" at school, but slip at times. Many students come into the school not speaking Korean and even one of the teachers, who had been teaching for 30 years, had just passed the advanced Korean language aptitude test at the end of the documentary filming.
In Seattle (and I suppose other cities) we have a very popular international language school. I think if (North) Korean schools in Japan became really Korean (and maybe include Chinese) language schools, and run more to promote Korean language and culture, they might catch on with the Japanese there. It's hard to say. Honestly, I think forcing Japanese to all learn English is not really helpful since Japanese schools pretty much fail to teach it.