Welcome to Gaia! ::

You dont have to go with your school there are tons of programs out there that you can do that are based in Japan. I bet you could even go during the summer and stuff. Most people go over there to teach english to students. Ive always wanted to do that. I have a lot of friends who have studied in Japan or who are there right now ( I lived in a language dorm where most people did exchange cause they were language majors) If you have to do it during highschool go during your sophmore or junior year. Those are the best times I think. Ok im rambling hehe.

oh i forgot to add that they give you a 6 month training program with japanese culture, speaking etc. and then you have regular meetings and they give you some information and stuff

Try looking through a local Japanese consulate's web page* to see what programs they have available. Also be aware that the Japanese school year runs April to March so if you want to attend during your sophomore year it might be wise to just end that year of schooling when the Japanese school year ends mid March.

*Think of a major city that you may live near and then check to see if there is a consulate. Some cities that do have a consulate: San Francisco, Los Angeles, Atlanta, Miami, New York, Chicago, Houston, Portland (Washington, I believe). There is also one in Hawaii. There are others too but I dont know exactly what city they are in. Washington D.C. holds the Japanese Embassy to the US.