Although elementary, junior high, and senior high schools are perceived as different, their differences are essentially ignored when organizational theorists characterize schools as loosely linked systems. Such systems share two characteristics: absence of shared goals and decentralization of power. To facilitate development of a more differentiated theory of school linkage, a recent study explored empirical differences among schools at three levels and attempted to explain these differences. Elementary schools consistently have stronger linkages than junior high schools, which in turn have stronger linkages than senior high schools. The data from a sample of 104 public schools in Pennsylvania and New Jersey suggest that differences between levels cannot be attributed to the staff's personal characteristics or to such organizational characteristics as size and complexity. An institutional perspective helps explain the differences between elementary and secondary schools in terms of size, staff, specialization, and gender composition. Historical evidence indicates that these differences result from institutional forces creating different expectations about how older and younger children should be educated. Four statistical tables and 59 references are appended. (MLH)