First off, welcome to WebmasterWorld. It's cool to see someone your age jumping in and thinking about your life and asking good questions like this one.
My strongest piece of advice is to develop a hunger for learning. Discover what sparks your passion and your curiosity, and then explore it. Ask questions, pull things apart, and really learn how they work together as a system.
If you're interested in search engines, that could take a lot of different forms. You might focus on math in school, to understand how calculations are done. You could study language and practice writing, if you're interested in content writing. You could learn about coding languages, if you want to learn about programming. Or, if you've got a creative flair, take art classes and become a graphic designer.
If you want to practice SEO, treat the web like a lab experiment. Create a site, build pages, and see what happens. Create A/B experiments. Choose a topic that you know and like. That way, you'll find it easier to create the site and the content you need. If you're bored writing the text, your reader probably will be bored and click away. So, again, follow your passion.
Very few classes in high school (or college) will directly prepare you for a specific profession. However, you'll learn the base-set of skills you'll need. You'll also learn how to ask thoughtful questions, explore alternatives, and find meaningful answers.
I'm a big proponent of life-long learning. School is one learning community, but places like WebmasterWorld are also excellent learning communities.
According to the American Booksellers Association, the average manager reads 0.7 business books every five years. However, the average Fortune 1000 CEO reads 7 business books every year. That's a 5000% difference! I know you said you were interested in SEO rather than being a CEO, but the point still holds true for any profession. If you want to be at the top of the game in any profession, one of the best ways to get there is to have a hunger for knowledge.
Welcome one of the web's best forums. Check out the great threads here in the WebmasterWorld library, read new threads voraciously, and ask questions along the way. You can have a whole lot of fun while learning here.
Lots of luck!