Recently, many institutions have started to offer online services via the internet. In this respect, it can be said that one of the most frequently accessed websites in Turkey is the "e-Okul" system offering services in the field of education. The current study aims at evaluating the "e-Okul" system used by parents, students and teachers, in terms of usability, identifying the factors affecting its usability and, later, offering some suggestions to increase its effectiveness and quality. In the study, the "e-Okul" system was evaluated by ten high school teachers and 95 students and their parents. The Usability Test was administered to the teachers. Components of this Usability Test included a Demographic Survey, a Task List, a Satisfaction Questionnaire and an Observation Form that were used as data collection tools. The parents and students completed an Attitude Questionnaire. The data obtained was analysed on the basis of effectiveness, efficiency and satisfaction criteria, which form the basis for usability. According to the results of the study, the users reported some design and navigation problems, especially regarding the pages enabling access to student information. Nevertheless, the parents and students were found to be satisfied with the "e-Okul" system.