A late-semester spike in coronavirus cases at Johns Hopkins University, spurred by recent social events, has some students pleading for the option to take exams online. After many months of strict health protocols at Hopkins, the campus in Baltimore has seen more than 500 cases in the past week and, according to its online dashboard, had filled all available isolation housing. The spike shocked some students because of the school’s international reputation in public health and its early and enduring warnings about the dangers of the pandemic.