Treasurer Andrew P. Kehoe, of the Bath Consolidated (first though eighth grade) School (michigan) Board of Education, sets off a series of firebombs, killing 38 students and two teachers.  Kehoe was upset at a school property tax increase that he believed was responsible for the foreclosure on his farm.

       NOTE: Nearly a century later this has remained the largest mass murder at a public school.

       [added 6/27/2022] Thanks to Chuck McGlawn and Bill Holmes for this entry.