Study abroad is absolutely the best thing I can recommend while you're in school. I went to Spain for 10 months, and even though it was tough going some of the time, it was so worth it. Top five reasons to study abroad while in high school and college (in no particular order):
1) Learn a new language. Besides impressing yourself with surviving the ramp-up phase (average time, 3 months), you get to meet a lot more people, and make more money in the long run.
2) Learn a new culture. You have no idea what you take for granted until you go to a country that works with different assumptions.
3) Much easier to do before graduating. Once you're out of school, financial obligations keep the vast majority of people from ever having the chance to study abroad. It's now or never.
4) Impresses potential colleges and employers. Studying abroad takes guts and initiative. But because you're still in school, you get room to mess up. Making it through a study abroad program helps you stand out.
5) A lot of people love accents. Once you get the language, they love listening to you speak it.
Bonus:
6) Hate your school or home? Study abroad gets you away for a bit.
7) Like to eat? You'll find some awesome foods that don't exist in your home country.
Everyone should study abroad at least once, for a minimum of three months, with as little exposure as possible to their first language. It's like twinking -- there's a grind to get in and gear up, but once you're past that, it's awesome.