There is no perfect solution. The best compromise is a program that allows trained, qualified and tested individuals to be armed on school property. The problems with an armed guard at every school include but aren't limited to: the added financial burden to already bankrupt schools and more importantly anyone intent on causing mayhem at the school has only to eliminate one easily identifiable deterrent. In all probability Dylan and Klebold and others of similar intent would have been even more pleased with an opportunity to do what they did plus add in taking out one single triple bonus score target of opportunity to their game. Some of the others who massacre would feel the same way.
Now, in the situation of fully qualified, trained and tested individuals being allowed the option of possessing protection on a school campus, there could be anywhere from a few to several to a dozen or even a couple of dozen authorized individuals. Nobody would know how many individuals are authorized out of the entire campus of individuals. To attack a school someone would have to consider it possible that every adult on the premises is trained and qualified to stop them. They wouldn't be wearing an identifiable uniform following a set schedule and pattern of movement making themselves easily eliminated.
It isn't a perfect solution. It's just the best solution.
Right, I was thinking of the pros and cons of having an armed uniformed security person or an unidentified individual that is armed. The uniformed individual would give an obvious presence that the school is being protected. This might deter an deranged person or possibly give that person an additional "score". They would most likely want to take this person out first so as to be unencumbered with their assault. The flip side would be a "secret" person in the school who is armed so the perpetrator wouldn't know who has a gun. The assailant might however choose to assault a school that doesn't have a known obstacle like a uniformed armed person, because they might assume the school doesn't allow guns on the premises. I guess if I had to pick, I would choose the latter. Have at least one, armed person, or have a gun available, secured on the premise at all time.