I have yet to understand why the U.S. does it differently.
I don't know about other places in the US but you pretty much just described the exact process that I went through for my education. The US in general does have some problems though mainly in the requirements in secondary language and arts. Also, it seems like there is a little too much emphasis placed on the individual motivation of the student to do well which makes a high school diploma relativly worthless in the job market. My diploma which constituted an education in calculus, music, basic computer programming, spanish, etc was equivalent to someone who finished their senior year in basic math and who barely knows how to read and write in their own language. The system allowed me to achieve more as long as I supplied my own motivation but did not
require much of anything substantial.
By the way, for a fun second-term drinking game, chug a beer every time you hear the phrase, "...contentious but futile protest vote by democrats." By the time Jeb Bush is elected president you will be so wasted you wont even notice the war in Syria.
-- Jon Stewart, The Daily Show