This discussion is about gun violence. Saying there weren't a lot of school shootings back then is about as helpful to the root issue discussion as saying that cyberbullying wasn't an issue before the 90s anyways.
No really, what makes them different, what's the obvious difference between the kind of person you picture robbing a store and the kind you picture committing a mass shooting?
Be bold, tell everyone the difference. Explain to us this essential difference that makes it so important that we segregate mass shootings from all other forms of gun violence as somehow a special sort of case?