Does reading a lot of Wikipedia give people a wrong idea how science works?

[FlatAssembler]

If there is a shooting it's obviously not happening somewhere that really gun-free now is it?

The point is that labeling a place a gun-free zone makes it a soft target. A would-be shooter can be almost sure he won't get shot there.

Also, many of these mass shooters pick places where the likelihood of being shot is extremely low - elementary schools, churches, long-distance sniper shoooting.

That probably also plays a role, but it doesn't explain why mass shootings are less common in schools where teachers are armed compared to schools where teachers aren't armed.