One of the largest school mass murders was a very creepy story. A disgruntled employee of the school built a bomb under the school and killed a ton of people. It’s a long time ago...1920’s or such? And it could have been worse if I remember, only one of the two bombs went off or something like that.
Curiosity made me look this up, you were right except it happened in 1927. https://qconline.com/opinion/columnists/marlene_gantt/school-massacre-deadliest-in-history/article_1394e75c-6c79-5ba6-82bc-d305ed3e7be4.html
And if you take away firearms that will indeed be the new popular way to kill in mass. The situation now is nothing compared to what it could actually be.