Virtual Reality provides the ability to immerse users in realistic environments, which enables utilization of the technology as an immersive educational tool. This is particularly useful for educational fields that require students to visit certain locations, or that concern hazardous situations and materials. The EIT Raw Materials Project MiReBooks intends to develop novel augmented and virtual reality teaching tools to mining education. Within the project, we developed an interactive multi-user VR environment, named MiReBooks VR, for teaching mining to students by simulating a VR mine and creating learning scenarios in it. In this paper, we briefly described MiReBooks VR, and then focused on determining the capacity of the server running in a head-mounted display by measuring latency. To assess the system's capacity to handle multiple students connected to a class session, client simulation tests of up to 30 simultaneous connections were conducted. The results suggests performance issues with respect to latency affecting all peers that could cause a negative effect to the VR user experience. In addition, the results indicate that the frame rate requirements for VR applications are difficult to maintain in multi-user environments using current off-the-shelf VR equipment. Based on the development experiences and the tests, we provide five lessons learned that can be of interest to software engineers and researchers working on the development of multi-user VR systems.