How to stop school shootings and mass murder

Responsible gun owners recognize an AR-15 (the military version is the M-16) as a good rifle. It is very good at killing lots of people. Remington manufactures a 90-round drum magazine for the weapon's .223 ammunition (5.56 mm NATO), which costs about $120.  Standard magazines are 20 and 30 rounds.  There is no reason to produce a 90-round magazine.
I qualified with the M-16 during basic training in the Army.  It is a fine weapon, but it was designed to kill lots of people.  The M-16 was not made for hunters; it was made for soldiers to kill the enemy.
Adam Lanza killed 20 children with an AR-15.
The unspeakable tragedy on December 14 in Newtown, Connecticut, in which 20 6- and 7-year-olds and six teachers died could not have been prevented.  This was the fifth school shooting this year since an eighth grade student engaged police officers at Cummings Middle School on January 4 in Brownsville, Texas, and was killed.  No one else was killed or injured.
There have been at least 73 school shootings since the 1999 Columbine High School massacre in Littleton, Colorado, in which 15 students and a teacher were killed; 23 wounded.  The gunmen killed themselves.
If we had a solution to this, I think we would have found it already.  It is sad commentary on our people and political leaders that we haven't.  We must try to keep deadly weapons out of the hands of mentally ill people.  But budgetary cutbacks on mental health services and institutions -- both at the state and federal level -- make it unlikely that someone who wants to kill lots of people will provide warning signs to a mental health professional before he strikes.
I'm afraid gun control won't work very well either, but reimposing the assault weapons ban, which expired in 2004, might help some.
Security doors for schools that require the person who wishes to enter the locked security door to push a buzzer or intercom and a camera trained on the entrance is certainly a low-cost approach.  These security doors are probably being installed in schools around the nation as I write this.  At least I hope so.
The only common-sense solution is to arm teachers and teach them how to use their weapons to protect themselves and their students.  Not all teachers, but at least the ones who volunteer to get the firearms training they will need.  And there definitely needs to be a semi-automatic pistol in the principal's office, which is usually found close to the entrance of schools.
It's either that, or post an armed guard at the front door.  Because how do you keep crazy people from getting guns and killing people?  As a nation, we don't have good options to resolve this.  Now the politicians will appoint "blue-ribbon commissions" and special committees of lawmakers to come up with suggestions to solve the problem, but new policies and laws and enforcement are not what we need.  We need to do something practical to protect our children in the short term, so this is what I think we should do:  Arm teachers.