Traffic Lights, Signs and Controls: Can a Line of School Buses Ignore Stop Signs on the Edge of School Property

My question involves a clarification for a law in Pennsylvania.

Hi, I am a high school news reporter that has noticed that a number of school buses at my school ignore the stop sign that is at the end of a school driveway where it meets a main road. The buses all leave in a line. I am trying to determine whether or not this is against the law. I belive the Pennsylvania law is a little bit vague regarding whether or not it is illegal as the stop signs are on private school property. I have barely found any information online to help me figure out the answer with the exception of this article from 2005 http://ift.tt/2ra9myn

Any help is greatly appreciated.

Traffic Lights, Signs and Controls: Can a Line of School Buses Ignore Stop Signs on the Edge of School Property