Obviously guns and more specifically automatic guns make it a lot easier. But are we just drawn to these events because of the obvious extreme tragedy, or because they are something new?
Crime committed with automatic weapons has been exceedingly rare since the passing of the national firearms act in 1934, which added more invasive background check, high fees, and mandatory registration of automatic weapons, along with several other classes of firearms and accessories. To the best of my knowledge, there has never been a school shooting in the US where an automatic weapon was used. (And this is the part where I leave myself some wiggle room, as some people use the term 'automatic pistol' to refer to a semiautomatic pistol, which is a separate entity from a machine pistol; the latter being a pistol capable of automatic fire).
With that out of the way, the earliest mass murder at a school that I've heard of is an indian attack on the Enoch Brown School, which happened back in 1764 and left 10 students and 1 teacher dead.
The Bath school disaster also stands out as an earlier example, this one from 1927. This one was done with explosives rather than guns, but remains the deadliest mass murders to take place at a school, with 35 people dead.
So the short answer is no, mass murders at schools is not a modern phenomenon, but one that dates back 250 years. I can't speak to the frequency or severity of these shootings without violating this subreddit's 20 year rule, so that's a question that will need to be asked elsewhere.
Violence is endemic in human society, so it's not surprising that violence also occurs in schools. The most interesting thing I have come across from the middle ages involves the University of Paris in the early thirteenth century. Here's the wiki on it, but it basically came down to a fight between the students and the townies (please insert modern parallel here).
Here are a few others that may interest you:
(1) Sara Deats. The Aching hearth : family violence in life and literature (1991).
(2) Dinah Shelton. Encyclopedia of genocide and crimes against humanity (2005).
(3) Norval Morris. The Oxford history of the prison : the practice of punishment in western society (1995).
Hope this helps a little. Happy Reading!