In recent weeks, Covid-19 cases have exploded among people in their 20s and 30s. Public-health officials say that’s partly because of so-called superspreader events, including gatherings of college students in off-campus houses, fraternities, and bars. Confronted with social-media posts showing crowds of students dancing and drinking together in defiance of local orders, colleges have been forced to zero in on what’s happening just outside their borders.