There is a huge scarcity of documentation of instances in which students do not follow the peer review training guidelines. One factor in these unanticipated scenarios could be learners' self-efficacy (SE). The current investigation illustrates how different sources of SE contribute to students' agentic orientations during peer review. For this purpose, six secondary-one students were paired to implement peer reviews in an after-school English writing course, after receiving peer review training. The data from three dyadic peer review sessions, stimulated recalls, and pre-/post-interviews were triangulated with quantitative data from 20 learners. The results showed that the students' low SE for self-regulation (SESR) for peer review at the outset overshadowed the impact of training and influenced the use of strategies by them during the peer reviews. Whereas those with high SESR followed the instructions from the training session and regulated the peer reviews professionally, those with low SESR ignored these guidelines, which resulted in constrained agency and promoted their skepticism of peer review in the end. However, by comparing their own performances as reviewers with those of their peers, the students' SE for regulating future peer review also changed. This paper underscores SE as an important construct in peer review for L2 young learners.