Is it realistic to believe that a teacher could effectively defend against an active shooter, using an AR-15, armed with only a handgun?

can I take down an assailant with an SR 22? damn right, pocket knife? I will try if forced too, the bigger question with all of the shootings , is why hasn't mental health come up, of the last 17 shooters all of them have been on medication ...... all the while the big pharma companies have given over 2.5 billion to congress while the NRA is around500 mil..... follow the money

I am still undecided about arming teachers to protect our schools. In all the confusion, it could turn into a wild west show with rounds going everywhere, possibly hitting innocent people and raising the casualty rate. Just to get out of the box for another idea, how about protecting schools similar to how the TSA protects airports. Regulated entry points, with screening equipment, and the ability to deny access by shuting the entrances down in an emergency

Rephrase the question: 'Is it realistic to believe that a homeowner could effectively defend against an intruder with an AR-15, armed only with a handgun?'
You betcha.

Yes, it is realistic.

Yes, it is, it definitely is. Two shots to center of mass. BUT, the question is will a fifteen-year Home Econ teacher respond to that shooting situation in the same way as a fifteen-year police officer? Pound for pound, skill for skill, experience for experience, I doubt it. BUT, that teacher is on site, and the cavalry may be several minutes away, and time is critically important. Anything to distract or deter a nut job shooter from popping more caps may save lives and/or delay his or her attack just long enough for the police to arrive and frag his or her completely worthless ass. And that is the what will serve the community the most, a dead shooter and as few the number of civilian casualties as possible.

The cop on site did nothing, he is on film. The police stood by and let it happen. One pistol could have made a difference. There are many acts of teachers saving students with their lives. A pistol would have given another option to just shielding someone with their body.

A lot depends upon the individual. One must be very selective when it comes to choosing whom to arm. It is one thing (and usually fairly easy) to train someone how to shoot well. It is another thing to find someone willing to kill another human being. Sometimes you can find someone willing to kill but they do not act on reflex, they stop to consider the situation first. Non-reflexive shooters are often killed by the assailent. Among the civilian population, most reflexive shooters are found in military veterans who have seen combat, the very self reliant individual, someone from the country or an area of low population density, someone from states south of Maryland, many NRA members, and from churches that stress self-reliance (e.g. LDS). Today's police officers regrettably are not as reliable in this area as they once were. Between all the critics that beset them and the "touchy-feely" requirements necessary to be hired, we are often unable to hire the best candidates. This is far from being every officer but there are many that are frankly "timid". NOTE for record, having been a police officer I realize that we are responsible for where every bullet stops and that criminals are not so encumbered. This is however not an excuse for inaction. Practice, practice, practice, until you can call your shots within a reasonable range for a handgun and shooting becomes instinctual. An examination of the behavior of the Broward Co. Deputy Sheriffs in the FL shootings recently will reveal gross timidity in action. Nevertheless a retired LEO or one who is making teaching their second career, are still a good bet. Remember that while killing the assailent is preferable (most being natural born cowards), most mass shooters only need to become afraid before they will seek self preservation. This is another reason why pursuit unto destruction (within reason) must be instinctual. These killers have plagued schools, theaters, malls, etc. for years. I used to only move about armed when deployed, now I do so everywhere. I am determined to protect myself, my loved ones, and those around me from these predators unto death. The teachers / administrators entrusted with this new responsibility must feel likewise.

Only with the Grace of God.

Yes, for two reasons.
1- In the past, all mass shooters either gave up or killed themselves when confronted with a armed individual. The fact the someone was shooting back at them totally ruins their game.
2-When the first cops arrive they will be armed with a handgun. While more and more average uniformed cops are beginning to carrying "patrol rifles" (usually a AR-15 type platform), most still are only armed with a handgun and a shotgun. But a handgun is still enough to stop the badguy (see #1).

However, in active shooting circumstances the key to winning is to have the proper mindset. Which is to be very aggressive and stop the threat right away. It requires John Wayne type of tactics which goes against normal police tactics. Teaching that mindset to teachers will be he tough part.

NO, I have seen how security local forces walk around the schools and supposedly guard our educational facilities and it is done very unprofessional. I don't need to go into detail about what I am saying, just go to a local school in your area and make an assessment of what you are looking at as security operations is concerned.