From Quora: "How do I test out of high school early?"

 Click here for my original Quora post.

Talk to your guidance counselor. If your school doesn’t one have those, speak to the principal, vice principal, etc., basically anyone who can let you modify your class schedule. I’ve known people who graduated from high school a year early by taking the classes most students take in the junior and senior year (e.g., chemistry in junior year, physics in senior year), all in the same year. That leads to a busy schedule, but you can do it if you work hard.
Alternatively, the school may let you take some courses in summer school. I’m fairly certain the high school I attended didn’t have any summer school courses other than driver’s education, and remedial courses students who failed the “normal” versions of those courses. The idea was driver’s ed was more of an “elective” than a real academic course, and summer-school English was meant for kids who failed English and needed that English course credit to move on to the next grade and/or graduate.
If you plan this early enough, you might even be able to start taking extra courses in your first and second year, thus making your third year less demanding, while still allowing you to skip senior year.
But your school might offer “real” (non-remedial) academic courses if there is enough demand for them. You can also see if your teachers/counselors/the principal can and will accept test scores as proof deserve course credit for a course you didn’t take in school, but obviously studied on your own, which is probably what you meant by “test[ing] out of high school early.” I know you can “challenge” an AP (advanced placement) test from the College Board, simply by signing up to take the test. Challenging a test is taking the test without having taken that course. If you get enough AP scores, or school credits, or things like New York Regents Exam scores, you can pass without get credit for courses in which you did not enroll. See, for example:
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https://apstudents.collegeboard.org/TL;DR: Ask your school’s principal, counselors, and teachers. Look into taking AP courses or whatever state, local, etc. tests you need to take to get course credit.