I'm not sure if it's laziness or people just underestimate self defense/martial arts. Using guns solely to protect yourself is a crutch, and a very risky one (mostly for other innocent people) at that.
Do you think a school shooter would be very successful if the classroom he entered had a teacher or student that could go all Sandy Cheeks kara-tay on his trifling butt the moment they walked in?
Disarm and overpower, not shoot down.
I agree with this post barring the bolded bit. Unless the teacher / student is standing right beside the door, I doubt they'd have enough time to run over to the shooter and disarm him before he fired. Unless he was exceptionally hesitant or got very close to his victims before firing, I don't think martial arts training is a viable solution to a gun shooting.