I don't know where you're located, but definitely check nearby colleges, univeristy, community colleges and various online distance learning programs (make sure they're through legit, accredited universities!). They often have special classes/programs/accelerated learner situations where high school students can take classes for credit. (And sometimes those credits can be applied to your college degree later on).
If nothing like that exists as of yet, don't give in. Talk to your high school guidance counselors and administrators and contact the college/community college administrators as well. If you are persistent I'm sure they will be able to arrange something to suit your needs. It may take more time and effort on your part, but if you persist a high school should (and most often will) support students who go above and beyond to enhance their education. It will be up to you to be proactive and to stand up for your educational rights, needs and desires.
Like I said, I don't know where you're located, but I think it's totally bogus for your school to require 3 years of a foreign language and then only offer one! Eesh...how about a few options on the menu people! :teehee:
Also...you may consider taking Spanish at your school (for convenience) and pursuing Japanese as an Independent Study or extra activity. Definitely don't give up on it just cuz it's not available--you'd be doing a disservice to yourself. The more languages the better! You'll be at an advantage when you apply to colleges, jobs, etc if you are fluent/conversational in more than other language. And Japanese will prove to be quite useful in the business and technology world.
Good luck. Keep us posted!