I would recommend you to finish school in your homecountry and either read at a languageschool or an university afterwards.

The arrangements i have done for me is that i will read at a languageschool for 1 year starting in October to see what i think about the country and learn as much as japanese as possible.
Afterwards i will return home and read at a university in Sweden for 6 months until i perhaps will return to Japan.

This plan is something i made up after discussions with a friend who read her last school year at a japanese school.
She already knew quite much japanese, booked thru an agency and had a host family so she was as prepared as you can be, but she still had a really hard time trying to keep up at normal classes, learning japanese and get a feeling for Japan.

I can't imagine spending 8 hours a day sitting in classes if I didn't speak the language. Japanese high school classes are mostly still in lecture style, with students rarely asking questions. I would probably be lulled to sleep after about 30 minutes.