The aim of the descriptive theory that is outlined in this article is to grasp the complexity of the school as a state and societal institution as well as single schools as organisations. The theory is based upon that a significant characteristic of this complexity is the ambiguity as far as the missions and goals – the outer boundaries – of the school-institution are concerned. Generally speaking it is reasonable to claim that the higher degree of institutional ambiguity that characterizes these outer boundaries the more extended is the available scope of action for single schools. To focus on the scope of action for single schools is also an expression of giving attention to the more and less extended latitudes for conducting professional oriented school development work.