Unified Modeling Language (UML) is a widely used modeling language in the software industry. However, students face difficulties while learning how to model complete and correct UML diagrams. In order to minimize these difficulties, educators perform individual and/or group modeling activities during class. Despite the importance of this problem, there are few experience reports comparing students' performance on these two forms of practicing modeling exercises in the classroom. In this context, this paper describes an empirical study aiming to compare the effects of modeling exercises carried out individually and in group. We evaluated the correctness and completeness of the UML diagrams and use case specification produced by the students and their perceptions about both forms of organization. The results showed that although the students presented difficulties in understanding the syntax, they modeled the use case specification and the class diagram more correctly individually. On the other hand, the students modeled the diagram and use case specification, and the sequence diagram more completely in group. Differently from what we expected, these results suggest that if the instructor's goal is for students to learn to design more correct and complete models, performing group activities not always show more positive learning outcomes. Instead, in some cases, individual modeling is likely to yield better results.