Unfortunately, many school districts do not have a lot of money to provide adequate security. In many other cases, money allotted for security is spent in a wasteful manner, leaving children and school staff at risk.
Many school districts spend a lot of money on emergency management, which is good. However, prevention (being proactive) is equally important. There are many techniques that can be implimented that are not very expensive.
In most schools, there is usually only one way in and that is usually through the main office. However, in many cases, office personnel are not properly trained and unable to stop someone determined to get in. And, the fencing usually is not that high, has two or more gates and is easily negotiated, and many schools have few to no cameras, and most cameras are not monitored in real time.
Access control and perimeter security are only two parts of many. Many schools do not have close proximity security aside from lockdown procedures, and in the event of an incident, the schools have to wait for law enforcement to arrive, locate and engage the suspect. The problem with this is that by the time law enforcement arrives, mucn of the damage is already done.
Security officers are only as good as their training and the policies implimented allowing them the full benefit that the laws extend to them. If not properly trained, especially those who are armed, it could create a greater risk rather than controlling or eliminating the incident.