This paper describes XRXL, an extended-reality system for increasing student engagement in large lectures. Students wear XR headsets to see 3D visualizations controlled by the instructor. The instructor can virtually retract the roof and walls of the classroom to allow for large-scale visualizations that extend beyond the physical boundaries of the classroom, or to turn the classroom into a 360° theater. The instructor can also partition the classroom into small groups of students and to assist individual groups as needed. XRXL was tested in an IRB-approved user study with 82 students in the context of a mock-lecture on neural networks. To the best of our knowledge, the study is the largest deployment of a co-located collaborative XR application to date. The study shows that students had a favorable opinion of XRXL, that XRXL had a low task load, an acceptable usability level, and that it did not cause cybersickness.