To quote Shakespeare, "Why do you address me in borrowed robes?"
The people on this site have an interest in firearms and doubtless most of them would meet the legal definition of an "expert" (i.e. someone who knows more than the average layman) but that hardly equates to any expertise relevant to your question.
I would say that hardly anyone on this site has sufficient knowledge of Federal law, State law in all 50 states, Federal Education regulations, State Education regulations in all 50 states to be able to render a sensible opinion.
So, what you're asking about is a volunteer group of un-trained or marginally trained people, outside the control of the school district and outside the control of local law enforcement, that are going to sit in private cars just outside the "no-gun" perimeter waiting for a call to come in to their cell phone to go riding to the sound of the guns.
- How does the volunteer organization propose to train the volunteers in how to respond to an active shooter?
- Who pays for that training?
- Who pays for the liability insurance to cover when one of the volunteers shoots a kid by accident?
- How does the school district ensure that such a force is fully staffed each day and ready to respond?
- How does the school know which cell phone to call?
- How do the police distinguish between active shooters and active shooter grandparents?
And what happens when all the grandparents who have been "playing" security guard get disenchanted with the idea of sitting in a car for 8 hours a day waiting for a call that never comes and stop showing up for their assigned rotation?
Bottom line is that it is an ill-conceived idea that does not reflect an adequate understanding of the diversity of relationships between local school and law enforcement establishments, 50 unique state systems and the complexity of the applicable Federal Regulations and would ultimately be a distraction/drain on the resources of both local school districts and local law enforcement agencies.