Whether law enforcement is likely to enforce such a violation is a different question than whether a violation according to statutory definitions has occurred. I am not versed in PA law and, therefore, cannot answer the question. However, I suspect that the answer will depend on the specific location of the stop sign(s). Does “private school property” mean that the property belongs to a private (ie., not publicly funded) school? If so, then it is possible that obedience to the signs is not enforceable – not sure if that is true in PA specifically, but it would be someplace to start your research.

If you are looking into this because you feel that there is an actual or potential safety concern, there is a likely simple solution that you could suggest to school officials. Someone legally authorized to direct traffic could be posted at the driveway to stop traffic and allow the buses to exit. This would likely not need to be a police officer. Trained flaggers or crossing guards typically have legal authority to direct traffic and allow vehicles to disregard otherwise enforceable traffic control signs and signals - again, I cannot state that this is specifically true in PA. But, that is something else a good investigative reporter could research.