COLLECT-UML is a collaborative constraint-based tutor for teaching object-oriented analysis and design using Unified Modelling Language. It is the first system in the family of constraint-based tutors to represent a higher-level skill such as collaboration using constraints. We present the full evaluation study carried out at the University of Canterbury to assess the effectiveness of the system in teaching UML class diagrams and good collaboration. The results show that COLLECT-UML is an effective educational tool. In addition to improved problem-solving skills, the participants both acquired declarative knowledge about good collaboration and did collaborate more effectively. The participants have enjoyed working with the system and found it a valuable asset to their learning.