CoLab is a collaborative tool for peer review of software programs for providing constructive feedback to first-time programmers. It targets two kinds of users: a trainer (who is the facilitator) and a trainee (whose programming skills have to be assessed and, if needed, improved). The tool provides real-time information on the reviewing ability of and common mistakes made by the programmers. As a result, it allows trainees to develop their programming skills without much assistance or intervention from the trainer. This article describes the design of CoLab and how it can be used as a tool for learning software programming in large classrooms. (Contains 16 figures and 7 notes.)