As Thomas Sowell would say, "There are no solutions, only tradeoffs." It's possible to make schools somewhat safer than they are now, but that option has at least some costs, financial and otherwise. You seem to be saying that this event was avoidable, but I wonder what it would have taken to avoid it. Schools already have security nowadays, and you'd probably have to impose prison-like security nationwide to have a serious impact on safety. I, for one, don't think that would be worth it.
Look at it another way: Swimming pools kill many people every year. I'm friends with a couple whose child died in a freak pool accident. Pools don't serve any essential purpose in our society, and it is certain that banning them would save many lives. Yet they are legal, and I'd bet that a national referendum to ban them would be voted down.
I can only conclude that Americans think the benefit pools provide(fun) outweighs the costs (many serious injuries and deaths). In this case at least, people don't want to make "safe" into "relatively safer".