I think the system varies from school to school. My high school covers grades 9-12, going freshman, sophomore, junior, and senior. I started middle school in 5th grade, but just before I graduated, they changed it so that the "upper elementary" included 3rd, 4th, and 5th grades. The middle school, or junior high as some call it, now has 6th, 7th, and 8th.
So, where I live:
Elementary: Kindergarten, 1st, 2nd
Upper Elementary: 3rd, 4th, 5th
Middle School/Junior High: 6th, 7th, 8th
High School: 9th, 10th, 11th, 12th
To each school its own, I suppose.
Oh, and here's some art: