Students' maturational levels, rather than grades or chronological age, should determine enrollment in the middle school. Diversity in maturation rates creates diversity in interests and attitudes. These differences necessitate the development of programs for the middle school which differ from the elementary and high school approaches. Two fundamental areas are grouping and climate for instruction. Separate but related dimensions are involved in student grouping. One facet involves the rate of student growth mentally; the other involves the rate of student growth physically, socially, and emotionally. A flexible school climate can be encouraged by staff attitudes that permit considerable freedom of action to youngsters and also by flexible scheduling. Related documents are EA 002 527 and EA 002 528. (MF)