You can take AP classes in high school, and if you do well enough said classes give you credit for certain classes in college. This means less time and a lot of saved money in college, because here in the United States college tuition is ridiculous and student loan interest is also ateocious. It's not unheard of to be in debt for 20 years after graduating. It also looks very good on college applications, which can lead to scholarships (where the college lets you attend for reduced or no cost).

Alternatively some school systems offer dual enrollment, where government subsidies allow you to attend college and take a few classes for free while you are still in high school. 

Put simply, the American school system is a ridiculous mess, probably stemming from high college dropput rates jacking up tuition costs, relatively unregulated student loan interest rates, and a general mentality that if you don't go to college you are stupid and will never get a good job, which leads to barely any people going to trade schools to be plumbers, electricians, and so on. It also means people choosing stupid college majors like women's studies (basically feminism as a class you pay for...as if being a certified feminist means anything) that won't get you a reliable job or have tons of competition for what few job openings there are.

Well, that turned into more of a rant than I intended :hmm