The purpose of this paper is to propose a conception of the learning process and to suggest its mathematical form. In this framework, the characteristics of the schools children attend are emphasized. The objective is to suggest a framework for the evaluation of research on school effects and a point of departure for new research on the impact of schools' instructional resources and the environments they provide on the amount that students will learn. Some findings of an investigation using this framework are presented, but the inadequacy of existing data available for the analysis make these findings suggestive. The main emphasis is on the conceptual issues underlying the model and the methodological problems that the proposed model presents. The objective is to propose a model that can ascertain school effects, if there are any. It should focus on learning that takes place as a result of students' exposure to school activities, i.e., exposure to classrooms, teachers, and textbooks. Further, it should focus on learning that is relevant for a student's further educational and, perhaps, occupational career, for the question that ultimately generates the interest in school effects is whether it makes a difference to a child's future attainment which school he or she attends. (Author/JM)