I suppose a school shooting could be defined as any gun being fired in or close to one of the liberal indoctrination centers, no matter how many shots are fired or what effect they have. In fact an even broader definition could be argued, since guns are not the only things that shoot, and not all schools are indoctrination centers (mostly only the ones supported by taxpayers).
But it'd be a good idea for a term used as commonly as this one to have a more specific working definition, at least from the perspective of the good guys. Liars, meaning freedom-hating liberal bigots in this case, love ambiguity, because it makes lying easier: you can say something using one meaning, while implying something using another meaning.
For example, using a broad definition, they could say that there were hundreds or even thousands of school shootings in a given time period, if they count rubber bands and spit wads too, and then describe one really tragic mass murder committed in a school with a gun. Cattle are then given the impression that the mass murder is more or less typical of the large number of school shootings given. Including spit wads is obviously is an exaggeration, but this method of deception by anti-gun bigots is not at all uncommon. And that's why the enemy of freedom likes to avoid real definitions.
It doesn't matter what's decided to be a working definition as long as it's used consistently. I would suggest this one: one or more gunshots, fired on or off any school grounds, that injures or kills at least one person on school grounds. But someone else might have a better idea.