The New Jersey School Boards Insurance Group has released a short analysis called "Lessons Learned from the Sandy Hook Tragedy." The Group recommends four steps to try to deter intruders. There's no link, so here they are:
1. The entry door accessed by the perpetrator should not have had a manual entry mechanism reachable once glass had been broken. Glass is not an appropriate deterrent to entry.
2. Walking the school grounds every 15-30 minutes to identify unregistered/unattended/ illegally parked vehicles or open entry points is requirement cited by both the US Departments of Justice and Education. The outside inspector should be equipped with a communication device (radio or cell phone) which could be used to warn staff inside the building to a potential threat. Any staff person is capable of this function.
3. The shooter prevented office staff from warning building occupants by denying them access to the PA system. It has been reported that a custodian verbally alerted building occupants of the impending danger. Unfortunately, this heroic action did not get the warning to all the building occupants timely. A back up plan for communication within the building is necessary. Possibly a panic button which sounds an alarm which informs the entire school population at once regarding the potential danger.
4. Finally, and most important, is a school security evaluation performed by a qualified school security company. A qualified school security company will employ a combination of experienced school administrators and security experts knowledgeable of the U.S Department of Justice and U.S Department of Education recommendations for preventing school violent acts.