In SC, this depends on the type of posting of the reduced speed limit. If it was a sign with a flashing signal which reads "speed limit reduced when flashing", then you would not be guilty unless the light was flashing. If it is a sign that says "school zone - speed limit 25", then that is always the speed limit within the area which is delineated as the school zone.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed to practice in the appropriate jurisdiction where the legal issue may be filed or in the state where the law applies.