If the city was truly interested in making this all about child safety, they'd invest in signage like this...
Put the controls to the lights inside the school and the Principal or custodian can flip them on about a half hour before classes start and off about a half hour after school lets out (or whatever times are appropriate based on the times that buses arrive and depart). If school is not in session one day, the lights wouldn't be switched on. If school was being held on a day or time that is unusual, the lights could be turned on to make people aware.
This would eliminate the whole "I didn't know it was in effect today" argument. If the lights are flashing, the limit is 30. If they're not, the limit is 50. No need for ridiculously long and unnecessary school zone schedules.