I don't think there ever will be. I would probably not shoot a kid that I only suspect would shoot someone. I doubt a teacher would either.
(Also, is the kid a "school shooter" if he/she did not harm anyone?)
There has been a case where a principal got a gun from his car and stopped a school shooter though.
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.
There have been interviews with bystanders who had firearms on their person during a mass shooting and they didn't use their firearm because they didn't want the police to think they were on the side of the attacker(s). The idea that life is like a video game where there are unambiguous and easily identified "good guys" and "bad guys" in a chaotic situation is very juvenile.
I think I agree with you: It is difficult to identify between good and bad guys. A kid with a gun might be a good person making a terrible mistake which is why I wouldn't want to shoot him/her, especially if that person hasn't harmed anyone.