https://en.wikipedia.org/wiki/List_of_school_shootings_in_the_United_States One of the worst decades for school homicides was 1900-1910. That list is far longer than I thought it would be.
And the perpetrators of those 1900-1909 list include Members of the school board Teachers (Read this one) School Directors Principals and all the other usual suspects.During an exciting school board committee elections, H. C. Branstetter attempted to murder H. B. Eastman. One shot was fired, but the gun was struck down and the weapon was snatched from Branstetter before he could shoot again.
Teacher Fletcher R. Barnett shot and killed another teacher, Eva C. Wiseman, in front of her class at a school. After shooting at a pupil who came to help Miss Wiseman and wounding himself in a failed suicide attempt, he waited in the classroom until a group of farmers came to lynch him. He then ran out of the school building, grabbed a shotgun from one of the farmers and shot himself, before running away and leaping into a well where he finally drowned. The incident was likely sparked by Wiseman's refusal to marry Barnett.
Two school directors got into a heated discussion over school business. When Director Samuel Egly threatened Director William Kling with an ax, Kling shot Egly through the heart, killing him.
E. E. Mangum was shot through the head and killed by the principal of the high school, Professor V. A. Alfonso. Mangum remonstrated with Woodward because he had administered a severe whipping to Mangum's 15-year-old son. Mangum finally lost his temper and shot Woodward through the wrist. Woodward was handed a pistol by a bystander and shot Mangum through the head.
I meant to refer to mass shootings. Most of the ones you reference seem to be either accidents or personal matters that were 'resolved' on school grounds. There was a graphic here a while back that showed a timeline of mass shootings. Definitely has gotten worse over the past quarter century.