From the researcher at Northeastern who wrote the USA Today op-ed; there are some nice graphics.
Mass school shootings are incredibly rare events. In research publishing later this year, Fox and doctoral student Emma Fridel found that on average, mass murders occur between 20 and 30 times per year, and about one of those incidents on average takes place at a school.
Their research also finds that shooting incidents involving students have been declining since the 1990s.
[…]Four times the number of children were killed in schools in the early 1990s than today, Fox said.“There is not an epidemic of school shootings,” he said, adding that more kids are killed each year from pool drownings or bicycle accidents. There are around 55 million school children in the United States, and on average over the past 25 years, about 10 students per year were killed by gunfire at school, according to Fox and Fridel’s research.