I did answer your question by the way. In the case where the killer is a student, and they know about a dare day, and they plan things in advance, then they would take it into account. In that very specific situation, which has never to my mind ever happened, you might have a point.
Which is also because places where people have ready access to guns are also places that wandering insane people don't have easy access to the locale.They both do and to not happen at places where people have ready access to guns, but they do NOT happpen where they DO have that access.
Mass shootings have nothing to do wtih the second amendment for either side of the argument. It has to do with insane people killing people and where they are more likely to be found.