There were in fact very few school bombings, even in the days when explosives were easy to get. This simply points to the fact that most people are not inclined to murder, and this appears to have been even more common in the "bad old days" when explosives and weapons were easy to get hold of. Why the change?
You mention the failed bombing at Columbine, which if successful would have caused far more deaths than the shooters did. If the shooters had not had firearms as their fallback plan, would they have kept working on their bombs until they were successful?
At Columbine, the easy access to firearms may well have saved lives.
You mention that "one of the most common characteristics" of the shooters "is their apathy towards innovation and their preference for using a credit-card as their sole means of self-arming" - but in fact the Columbine shooters did attempt a bombing. Other common characteristics of the shooters are that at the time of the shootings they were either on or had recently been on powerful psychiatric drugs. They also tend to have progressive political tendencies - just like the Weathermen.
Naturally, conservatives look at this and see their self-identified enemy repeatedly attacking them, and then demanding that conservatives disarm in order to prevent more attacks. Do you wonder why they don't buy it? They have not forgotten that socialists disarmed their victims before committing genocide in Germany, the Soviet Union, China, and other nations.
The idea of arming teachers is not merely a deterrent. It is also a means of actually stopping armed attackers before they have maximized their killing. The church shooting in Texas was not stopped by an armed citizen, but the shooter was prevented from taking his campaign any further.
Of course, it is also worth noting that in the early 1990s there were claims published that U.S. intelligence services hoped to disarm the public by staging school shootings in order to persuade the middle class to give up their weapons. Very curious that shortly after those claims were made, such shootings started to happen with regularity.
Frankly, it looks an awful lot as if the terrorist groups that waged a campaign against the people of the United States in the 1970s simply decided to take a break until they figured out what they had done wrong, but have since infiltrated the government and are now waging their campaign of terror from the inside. That does not incline me to give up all means of self defense. We have seen their work before, and we know that they are liars and murderers.