Just remember that set designers at LEGO aren't all fun and games, there is a lot of work put into each set that most people don't know about. Your best bet is to start out by researching the job, it's requirements, and get as much info as you can about it. After that, think of what courses in high school and college would best prepare you to take on that job while still being flexible enough to provide alternatives if you get turned down. LEGO has been losing sales recently from what I've heard, so they might be cutting off jobs instead of creating them, so be ready for anything.
Always take courses that will provide backup employment unless the field you're entering is in high demand, i.e. health care or engineering. I have no clue what courses would be best, but anything having to do with art and graphics design would definitely help. But make sure you know what the occupation is like to the letter before making course choices, as it can end up wasting a considerable amount of time if you change your mind.