I'm an AFSer, leaving for Paraguay in 29 days (not that I'm counting or anything ^^).
Grades almost never transfer from your year abroad. Occasionally credits will, but almost never grades. Try to graduate early if you can, or do your year abroad as a gap year. Also, the deadlines are rapidly approaching for the AFS programs that leave this summer/fall. The application is equivalent to a college application, and takes a while. You're also going to need doctors appointments, interviews, letters of recommendation, and such. I don't know what you mean about years of school "counting." My district goes on credits, not years, as I believe most districts do. I don't know if you can be gone for second semester senior year and still graduate. Like I said, graduate early, or go for the first semester of your senior year. Also look into summer programs if the school programs don't fit. What year of high school are you in now?
pleasepleasepleaseplease don't be so set in your country choice. There are tons of great "off the beaten path" countries that you may never get to live in again, and most of them have GREAT scholarships available. Some of the main things they look for in your application and interviews are flexibility, adaptability, and open-mindedness, and being dead-set on one country does NOT show this. There are only a certain number of people that can go to each country from each country due to government restrictions. Nearly every university offers programs to Japan, but when are you going to get the chance to go to Malaysia or Finland or Honduras or Tunisia again?
http://www.cultures-shocked.org/Joomla/forum/index.php is a great forum for exchangers.
Also, check out RYE, YFU, NSLI, or LPI. I'm obviously not as familiar with them as I am with AFS, but people on cultures-shocked will be.