A frequent task in educational settings is to assign students to groups based on stated preferences for some projects or topics. This paper introduces a web-based tool supporting both the work flow of collecting student data and the group formation. The latter is based on finding optimal solutions to suitable mathematical assignment problems, allowing for a number of constraints regarding size and structure of the groups. Evaluation results show advantages compared to manual procedures in terms of time savings for lecturers, and higher fairness and correctness as perceived by students. (Contains 3 tables and 9 figures.)