I'd recommend sticking with Spanish next year. If you've already started, might as well finish. Also, it's better(?) to be near-fluent in one language and okay at another than to not know much in either. Japanese is one of those languages where you could probably teach yourself and be relatively okay at it. Thank you, the Internet and your vast span of knowledge.
(I had a similar discussion with my parents about switching from Spanish to German, and they said Spanish is just more useful and more widely-spoken... but that's at least partially subjective.)
You're lucky that your school has classes for Japanese, though. At least there's the opportunity.