May I suggest the possibility that, when the daughters of the President are attending a school, the security is actually managed and dictated by the Secret Service and all armed personnel are ONLY Secret Service. IMHO - When the Secret Service is guarding a principal and the only other person in the room or area is a local cop or security guard, that cop or guard is then considered, by default, the greatest threat.
You may be right. What is certainly true is that there are enough kids of the rich and famous there, that friendly guns are always nearby. This article ,even if technically correct, is misleading.
All of that is probably true.