The largest school massacre in US history[0] happened in the 1920s. It was accomplished with high explosives. There were several school bombings in the 1950s too but few shootings. For whatever reason, school shootings displaced school bombings in recent decades. It has been a long time since there has been a major school bombing.
Bombings were the popular mode of creating mass casualties 50+ years ago even though actual machine-guns were widely available back then and almost completely unavailable for the last several decades.