At risk of other-optimizing:

How To Get The Most Out of High School

Take as many AP tests as you can. (in the U.S., at least. I'm not sure what the international equivalent is). Take AP tests even in subjects you haven't taken in school. You get credits towards college, it looks impressive on a college resume, and you get to go at your own pace. Specifically, microeconomics is very easy to teach yourself and very useful.

Take as many science and math classes as you possibly can. If your school limits you to one or two science classes simultaneously, go and talk to your counselor. If you make enough of a nuisance of yourself, you can usually get what you want - and no matter what you want to major in, foundations in science and math are important. If you do poorly in math/science classes, try doing all your homework three times - the day before the material is covered in class, the day after the material is covered in class, and again before the test.

Join clubs/sports teams. Even if you're introverted (especially if you're introverted), you need regular IRL social interaction to be happy. If you're at all athletic, sports are an automatic boost in social status in addition to being relaxing and sometimes fun. If you don't think you're at all athletic, you probably haven't found your sport yet - I have no hand-eye coordination, so I run cross country. Debate teams are also a great place to find people with the 'taking ideas seriously' gene, though their effect on rationality is mixed at best.

To make time for the above: figure out which classes you care about. Do your homework for those classes during your other classes. Shakespeare isn't really my thing, so I usually finish Physics during English. Gamify things: assign yourself challenges (get through ten math problems in History without making the teacher suspicious) with rewards for finishing them (no math homework tonight!).

Figure out what skills you are learning that you will want in real life, and actively optimize them. The ability to write coherently (and quickly) is important even if you plan to be an engineer, as is the ability to read and absorb stuff quickly. The ability to solve math problems without making stupid sloppy errors is equally important, though I haven't figured out how to optimize this yet. Also, school is a built in opportunity to practice explaining singulatarianism/transhumanism/cryonics to other people. It doesn't matter if people think you're weird, you won't see any of them again anyway after you graduate. Practice different approaches to explaining ideas you are passionate about and see which one gets the best response.

Apply for scholarships. There's tons of them out there, and many of them don't get enough applicants. It's free money, which you will need if you intend to attend a good college (especially in the United States.) And you should plan to attend a good college, unless you have an absolutely brilliant idea: graduates from expensive big name schools earn lots more than from cheaper schools.