Your third paragraph really stands out to me. I think you're falling victim to your ignorance of firearms. At the risk of sounding morbid or insensitive, an autoloading shotgun or semiautomatic handgun would be probably as effective in a school shooting as a semiautomatic rifle, the handgun possibly more so. Both of those have been in production and readily available to citizens since well before the World War 1.
One of the biggest reasons handguns aren't used as much is because many big box stores that sell cheap guns, Wal Mart for example, make it company policy not to stock handguns.