Perhaps you could link the article Pete and we can find out what evidence the writer bases their claim that in most attacks the shooter had been a drug user. The typical profile of mass shooters I've seen suggests a history of pot use but not other drugs - domestic violence history has been a lot stronger correlation.
There were historical school attacks, 1890 and 1910s were particularly bad.(Source
https://en.m.wikipedia.org/wiki/School_shootings_in_the_United_States)

School deaths and injuries by shooting in the United States by decade
Haven't seen anyone put into context what you would have expected with general population growth but it certainly feels there has been an increase in recent decades. Doubt it is due to any one single factor although iirc quite a few shooters have said their "inspiration" was Columbine.