Artillery accuracy isn't effected by daylight and it isn't just about daylight helping the evacuation, it is about how hard an unexpected evacuation would be if people had already gone to sleep. During winter you might do one at night if you felt it was easier to do the evacuation when most people would be at home but still awake but odds favor it would be easier to evacuate workplaces and schools, person for person, then homes. Most already have safety drills and I don't know but I am guessing in Korea those include 'WWAARR!!!' not just FIRE!!!. If I were them and if I had been doing school drills I'd want to do it during school hours on a school day. Even without the drills, far easier for a police officer or two and the teachers to coordinate several hundred kids then several hundred individual families.