Effectiveness versus affordability versus acceptability
Effectiveness, affordability, and acceptability are difficult tradeoffs and, occasionally, a seemingly ineffective solution to a security problem is chosen because of a lack of funding or pressure from the community to do something.
Although many effective security measures are too expensive for schools, cost alone is not often the ultimate driver. Most major changes to security policies, including the introduction of technologies, are often brought on not by foresight but as a response to some undesirable incident.
This is not to say that a good argument should be made for applying every physical security approach in every school. "Appropriate" preparation is, by far, the greater "art" in security system design, and it includes an evolving plan, beginning with defining a particular school's risks.
Research Report: The Appropriate and Effective Use of Security Technologies in U.S. Schools