Online learning platforms such as MOOCs have been prevalent sources of self-paced learning to people nowadays. However, the lack of peer accompaniment and social interaction may increase learners' sense of isolation and loneliness. Prior studies have shown the positive effects on visualizing peer students' appearances with virtual avatars or virtualized online learners in VR learning environments. In this work, we propose to build virtual classmates, which were constructed by synthesizing previous learners' messages (time-anchored comments). Configurations of virtual classmates, such as the number of classmates participating in a VR class and the behavioral features of the classmates, can also be adjusted. To build the characteristics of virtual classmates, we propose a technique called comment mapping to aggregate prior online learners' comments to shape virtual classmates' behaviors. We conduct a study with 100 participants to evaluate the effects of the virtual classmates built with and without the comment mapping and the amount of virtual classmates rendered in VR. The findings of our study suggest design implications for developing virtual classmates in VR environments.