"material technologies and methods would make this relatively inexpensive"
Marvin, I understand your point, but I think you are underestimating just how broke and how skin flint cheap the citizens in my area are. Just at the middle school, which is right behind my house, there must be 50 ground floor windows and at least 8 entry points. Even $50,000 to install safety modifications would be hard to sell, when that school can't even afford a librarian, a nurse, or a full time guidance counselor.
Too many people, especially in small towns far away from large cities, just don't believe that something like this could happen to them. After all, there are more suicides in my county than murders ( I actually heard someone say that). People here think that because everybody knows everybody else, and everybody has lived here forever, there are no people who would commit such a terrible act. I know that isn't reality, but people desparately want to feel safe. Installing bullet proof glass would mean acknowledging that they really aren't any safer here in Carmi, IL than the people who live in Chicago or New York.