But there are basically six general groups that seem to be most common. During the 1990s, studies of national patterns of grade organization show that more than half of the schools fall in the K-8 category or the middle school category. This represents over 40 percent of all students of middle school age. By the 1993-94 academic year, over 13,500 schools in the United States had been designated or categories as middle level schools. Psychologists and educators generally agree that sixth graders are more like seventh graders than fifth graders because of the early onset of puberty in most sixth and seventh graders.
Fifth graders tend to behave more like children than like early adolescents.
Ninth graders tend to behave more like tenth graders than eighth graders in terms of physical, social, emotional, and intellectual maturity. Some joke and say that nobody really likes ninth graders and, therefore, ninth graders tend to get pushed up into the senior high school whenever a school district is considering grade sequence for a proposed new middle school or transforming an existing junior high school into a middle school. This is still an area ripe for research by adolescent and child psychologists. Sadly, curricular and instructional reasons for developing a middle school program often seem to be cited as secondary criteria. In many states, aside from general standards and administrative guidelines, there is no specific or mandated middle school policy.