This horrendous tragedy was indeed, among the worst school shootings in history. It was not, however, the worst school tragedy as a result of terroristic acts, which includes one about which your average American has probably never even heard.
That occurred in Bath Township Michigan in 1927 wherein a disgruntled school Board member took at the least, several weeks to months to surreptitiously wire the entire school with Dynamite and another explosive.
The bomb was detonated by a timing device when the school was in session. Emergency responders were arriving when the bomber pulled up in his car that was packed with shrapnel and a case of dynamite and detonated that as well killing among others, the school superintendent.
When the carnage ended, 38 students and 8 adults were dead. If there is a saving grace in the story, it is that a lot of the explosives wired throughout the basement of the school failed to detonate and only one section of the school actually blew up. Had all of the explosive detonated the results would have been far far worse.
As many have alluded, we obviously have failed to learn that there is unspeakable evil everywhere, even in our own country and it doesn't just have to be by shooting. I think the above referenced incident and the Murrah Federal Building bombing in Oklahoma City in 1998 underscores that fact.