In our quest to ensure public safety and security, we have come to recognize that all targets are not equal. One distinction of significance is that of a 'hard target' vs. a 'soft target'--with good examples of soft targets being schools and airplanes.
Schools are 'soft' in that they are heavily populated with largely unprotected young individuals, security is sparse, and access is easy--pretty much guaranteed by law for public institutions. Airplanes are 'soft' because at altitude, they are pressurized into being flying bombs, that a pinprick in the skin can rip apart--think: shaking a soda can and pulling the tab.
So in the case of airlines, we mitigate. Access to the craft itself is restricted to screened and trusted individuals. Every individual who boards an aircraft is screened to determine if they are a threat. Every piece of luggage is screened for explosives. Yet still, things go wrong, as with the recent takedown of the [link url="http://www.theguardian.com/world/2015/nov/04/bomb-brought-down-russian-airliner-egypt-analysis" title="Russian airliner"].
Schools are another matter. Functional mitigation of active shooter or bomb placement events does not yet exist. How does one prevent an individual from carrying a gun onto campus? It's an issue that's further complicated by mitigation strategies that actually encourage more guns on campus in the name of self-protection. That strategy has obvious flaws, but nonetheless has traction in some states.
What is the answer? That's your challenge in protecting the public that you serve. Increased policing? Expensive. Metal detectors at restricted access points? Really expensive. Yet we can't and shouldn't put this issue in the 'too tough to solve' category. That would be irresponsible.
So get together with your colleagues and figure out what would work for you in your circumstance. Lives are entrusted to us as public servants, and we must fulfill our obligations.
As food for thought, consider [link url="http://www.amazon.com/Managing-Unthinkable-Preparation-Response-Leaders-ebook/dp/B015YFJCDG/ref=mt_kindle?_encoding=UTF8&me=" title="Managing the Unthinkable: Crisis Preparation and Response for Campus Leaders"], edited by Gretchen Baitaille and Diana Cordova (2014). This collection of case studies provides many ideas that may assist you in your search for your own best practice.