If you asked me to choose one thing the UK still does better than any other country in the world, I would say it’s the private boarding school industry. Centuries of intense competition between these institutions have led to high productivity, streamlined processes, and interesting managerial practices. While at first you may be free to explore and study any subject you like, by high school your ability to choose the most prestigious or in-demand subjects depends on your track record in earlier years. If you’ve studied them well and achieved good grades, you’re allowed to take them.
But it’s not just about subjects. Even the roommates assigned by the school often depend on academic performance (though this isn’t formalized and schools would likely deny it if asked directly). If you’ve done well in previous years, you’ll most likely be placed with the best-performing and best-behaved boys; otherwise, you get what you deserve.