It was a bombing, not a shooting, but the first school massacre took place in 1927. And it is still the deadliest.
To yours add this: The earliest known United States shooting to happen on school property was the Pontiac’s Rebellion school massacre on July 26, 1764, where four Lenape American Indians entered the schoolhouse near present-day Greencastle, Pennsylvania, shot and killed schoolmaster Enoch Brown, and killed nine or ten children (reports vary). Only three children survived.
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