Most people think you have to do four years of high school before you can go to college, but there are ways to start college earlier.

Bard College at Simon's Rock is specifically focused on accepting students after 10th or 11th grade.

The Shimer College early entrance program, which also targets students who have completed the 10th or 11th grade, is the longest-running such program in the United States.

Large list of early-college programs

Can't Complete High School? Go Right to College - New York Times article.

In many U.S. states, you can drop out and get a GED before age 18. This is another path to get into college. It helps to submit high scores on entrance exams, but you can always start with a community college or open-enrollment institution and get your degree there, or transfer later.

Many school districts offer programs where some classes can be taken at local colleges, and count for high school as well as college credit. Most schools also offer Advanced Placement high school classes that award college credit.

College-level learning can also be done prior to official enrollment, and turned into credit via CLEP, AP or other exams.