Why do you want to graduate from a Japanese HS at all? Just to learn the language? It'll help with some slang and colloquialisms, but to reach JLPT 2 or 1 will simply require study. If you plan to attend a Japanese HS, you will be expected to participate in classes and clubs, even though you can't understand the language. What is this going to do for your academic education? That is, if you can't understand the math or history teacher, how do you expect to learn math or history (and perhaps even get credit for them)?
Foreign exchange students that I knew who stayed 6-10 months and started from scratch (mostly) studied Japanese classes at least 2 times a day. They were expected to attend other regular classes, but they were not expected to do any work in them. They couldn't anyway, even when they had attained JLPT 3. It simply wasn't enough. And, their credits in those classes didn't even transfer back home for obvious reasons. Even if you plan to stay longer than that, you are basically going to be sitting dumbly in most of your classes, getting nothing out of them, and wasting time.
Think this over and find some intensive study program instead.