Mass shootings have been a recognized phenomenon in the US since Charles Whitman went up the tower at the U. of Texas with a rifle in 1966 and entered Baby Boomer lore.
That’s about as far back as I can remember, so I can’t tell you if these kinds of horrible events happened just as regularly before 1966. Those who have studied the question come to differing conclusions. For example, in 1927 a vengeful Michigan school-board treasurer blew up an elementary school, killing 38 students. But as spectacular as that must have been, that was before television allowed the entire country to watch news stories live. So the bomber didn’t become famous like Whitman. (That phase change in news coverage is one reason that Baby Boomers such as me naturally assume that history more or less began sometime between the JFK assassination and The Beatles.)
It’s possible that even more horrific mass murders are lost to history: Fires were major killers until not that long ago. For example, the 1958 fire that killed 92 students in Chicago’s Our Lady of Angels elementary school may have been the result of youthful arson.