I spent a year in high school in Japan. Easily the best thing I ever did, since it really set things in motion for my life.
I went on a program through AFS. I received
no credit for that year, so technically I spent 5 years in high school. But your school counselors may be different. It may be the case that you have to write papers once a month on your experience and that will count as something. It may be that you have to actually pass your classes in Japan to get credit. It's between you and your school. In my case, I just decided to not worry about it at all and focus on the experience rather than the class credit. So I wasn't really bothered when I got at most 5/100 on all of my math tests in Japan
Obviously there are going to be language difficulties. I took Japanese classes in high school for 2 years before going, and that was not "enough." To give you an idea, I took the JLPT level 2 at the end of that year in Japan, and I didn't pass. There was no formalized language instruction as I was literally the only foreign student at my school (1 of 2 foreign
people at the school, since there was also a JET teacher). AFS supplied me with a self-study textbook that was appropriate for my level. During kokugo class I would go into the library to study that textbook (sometimes with help from the library assistant, sometimes not), since kokugo would be 100% useless for me at that point (and probably even today...).
Throughout the year, I took math, history, physics, calligraphy, home ec, health, PE, english (reading), and english (grammar). As expected, I was frequently called upon to read from the textbook for the English classes.
Japanese high school can be like anime and manga at times. Devotion to certain social groups, putting in a lot of hours after school at club activities, rote memorization of facts and figures, cliques that put on fronts, surprising personalities... they're all present in anime and manga to some extent. Obviously real life is very different but as anime/manga are sometimes based on realistic concepts, you can find some strong correlations. If anything anime/manga (ones about high school life at least) could help you get better prepared for the lifestyle.
From a financial standpoint, there are a lot of different options. Ask the program provider about financial assistance, they can give you plenty of choices. If you're at all associated with Rotary, certainly ask them; they're constantly sponsoring students to do study abroad in both high school and college (but keep in mind you can't get a Rotary scholarship if you've already lived in the country for 6+ months).
Let me know if you have any questions...