Calculates average tenure of superintendents in 25 major cities since 1900. Superintendents' length of service has been decreasing, but not as precipitously as the press proclaims. A complex mix of environmental, local, and professional factors is responsible. The 1990 average of 5.76 years raises questions concerning the relationship of "short tenures" and urban districts' problems. (75 references) (MLH)