Well, if you`d be an exchange student, it might be difficult to get into a public high school without going through some sort of system.
For a private school, if you can pay for it they`ll pretty much let you in.
Either way, if you are expecting credit for that year, you`ll NEED to have a very high level of Japanese. Otherwise, it becomes an uncredited, but excused, year away from school. For universities, if you`re not there to study Japanese, you have to have JLPT 1. For high schools, I believe it is JLPT 2, but I may be mistaken. Either way, you have to be at a high enough level that you can actually complete the school work (in Japanese) in order to receive credit.
Edit: And I forgot - the basic academic standards are VERY HIGH. Expect very very difficult - as in high level
university level stuff - at the senior level of Japanese high school. I`m not sure what country you`re from, but a 2nd year middle school student here is studying the same stuff that my friends studied in their senior year of high school in the US.