Classes begin Tuesday at the University of Virginia, and students who returned to campus over the weekend learned of grim news to start the academic year: Authorities say a female student was sexually assaulted at knifepoint by an assailant who remains at large. The university’s chief of police distributed a message to all students about the attack, which occurred early Saturday in a residential area near the Charlottesville campus. Due to a technical glitch, the message did not reach the public flagship’s newest students, members of the Class of 2020, until 21 hours after the attack.