I had to do an internet search. It was Pearl High School, and it looks like the principal only stopped the subject from leaving the school grounds. Who knows if the subject would have killed more elsewhere, the subject had already killed that day even before going to the school.

That's the thing, we'll never know how many shooting deaths were prevented by "a good guy with a gun", because we don't know if more people would have been shot and died if the shooter wasn't stopped.