In the
American educational system, "middle
school" is between
elementary school and
high school, and roughly defined by the 10-13
age group. Some school systems completely
lack this level, running elementary school to sixth grade and starting high school or "secondary school" at seventh grade; in other places, it's called
junior high. The most common grade assignments seem to be 6th-8th grades, but arrangements exist that use virtually any 2-4 year subset of 5th-9th grades that includes at least seventh grade.
Middle school is a traumatic experience (or, more simply, hell) for most kids. There are two major changes that cause most of these problems: the adjustment to a class-shifting schedule, and puberty.
Let's tackle class-shifting first. In elementary school, a child is assigned to one homeroom ("Mrs. So-and-so's class") where he/she spends the majority of the day. For special subjects like physical education and music, the entire class is marched off to that teacher's room together. Usually by fourth or fifth grade, mathematics will be broken off into a separate class assignment based on academic achievement, so the class will split up for perhaps one hour a day. Students still primarily identify with their respective homerooms, though. This all changes in middle school: students move from class to class, spending 45 minutes to an hour in each single-subject class, and with totally different people in many classes. It takes kids some time to adjust to this task-switching and lack of a "home" within the academic environment.
Puberty is the other fun part. Each child gets his or her blast of hormones at a different time; dealing with this(or, rather, learning by spectacularly failing to deal with this) is what makes middle school painful. Cliques form and re-form at the drop of a hat; bodies are changing in new and sometimes scary ways (says the 1950s instructional filmstrip), and no one knows quite how to deal with those mysterious people on the other side of the gym floor at a dance. "Dating" can last a whole day and be one of the longer "relationships" among a particular peer group.
The only positive part of middle school for most students is the knowledge that high school is coming, and it can't possibly be worse than what they're dealing with now.