I suppose I can't get away without talking about this.
We all know the facts, such as they are known at the moment. A young man took guns that his mother had purchased, killed his mother, and went to the school where she worked. When he was not "buzzed in", he shot out the window, entered the school and went on a killing rampage, killing 20 children and 6 adults, including the school's principal, before committing suicide when the authorities arrived. I won't use any names, because, really, they don't matter, we're talking about bigger issues.
The shooter, some 24 years old, reported to be possibly autistic with "Aspergers syndrome" (the fancy diagnosis for intelligent but strange), was the product of a broken home, estranged from his father and bother and reputedly a big "gamer."
What can can be done? Honestly, I don't know.
Aside from the constitutional objections, gun control won't eliminate the possibility of such occurrences. Literally tens or hundreds of millions of guns are already in circulation. They last essentially forever if maintained correctly, and even a total gun ban was put in place, and confiscation ordered, it would be result only in the removing guns from the hand of the lawful. As they say, if guns were outlawed, only outlaws would have guns. Anyone determined to kill multiple innocent children, and commit suicide would likely find a way to get a gun.
Tighter screening for gun owners? Pretty tight screening already exists when new guns are sold. The fact is crazy people conceal the depths of their disease all the time, and such screening will be only partially effective at best. As with all such programs, the cost and invasiveness of the program will increase exponentially, while the effectiveness will achieve diminishing returns. The point at which the costs are worth the benefit will always be a matter of debate, and a bone of contention. In this case for instance, the purchaser of the guns passed the screening, and should have, unless one of our new criteria for rejection would be motherhood with a weird child.
Armed guards at all schools? There are approximately 98,000 public schools in the US. Let's round that to 100,000. Assume that each school would require only 1 armed guard, no allowance for sickness and vacations, etc, and that each guard costs $50,000 a year (remember, salaries are much less than total cost in the average governmental institution). That's approximately $5 billion per year to save maybe 50 lives a year, tops or $100,000,000 per life saved. I'm not sure what the accepted price for a human life is these days, but I know it's a lot less than that, by a least one order of magnitude.
At some level you simply have to assume that life is not perfect, and that you cannot hope to eliminate the possibility of tragedies, and do the best you can to design a society that allows freedom, and minimizes the possibility of such tragedy, and maximizes the chance of stopping it should it occur, within a reasonable use of resources, while maintaining optimal amounts of personal freedom.