Even though it goes against my libertarian inner self, it is time to start mandatory military training. Easy to implement: turn 18, go to basic training. Want to go to college? Go to basic training upon graduation as an officer trainee or as an enlisted if you drop out.
Make everybody go, no exceptions for any reason. No religious exemptions or handicap exemptions, simply everybody goes to basic training and weapons training. Make it optional to further train in military jobs or become an active member of the armed services.
This would solve several societal problems. Self discipline would be the biggest benefit, then comes the ability to defend each other when crazy people decide to kill their fellow human beings.
It is past time for the government to stop ignoring the fact that there are crazy people out there and to do something to protect our society from those who wish us harm. Enough of all the talk and promises, take action and do it now.