In California, there are two exams that may be taken to graduate high school early (or late). The GED may be taken by students 18 years of age or older and the GED diploma is equivalent to a high school diploma. (I'm sure you know about GEDs.)
Aside from the GED, there is the CHSPE (the California High School Proficiency Exam). You may be as young as 12 to take the CHSPE (effectively bypassing high school). The difference between the CHSPE and the GED, nevermind age requirements, is that the former requires guardian authorization for you to leave school if you pass the exam whereas the GED does not.
There is also a special program that some folks are in. I was "friends" with a guy who was in this program. He was taking college courses in middle school (grades 6-8) and was required to complete a single semester of high school. He's 19 now, owns a $4m house with 3 other guys, and earns a six figure income as an IT/Security expert. Bleh, but he's a dyckhead.