This paper explores a new approach to engage students in authoring educational content. This approach was implemented in AnnotEx (Example Annotator) system, which allows students to annotate computer programming examples with line-by-line explanations and review annotations produced by their peers. A controlled study of AnnotEx presented in this paper evaluated the impact of the community peer-reviewing process on the quality of produced annotations and student learning. The study confirmed that community feedback increases the volume and the quality of produced annotations and positively affects the work of weaker students. The peer-rating process enabled the community to distinguish good and bad annotations. Peer comments provided efficient guidelines for improving annotations and caused a significant increase in quality.