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

Yes, there was a police officer on scene. One who failed to do his job and respond according to protocol, remaining outside while allowing an unarmed civilian to enter. Why? I don't suppose we'll ever know for a certainty. One thing that we can be sure of is a good guy has a heck of a lot better chance of stopping a bad guy with a gun if he has one too.

Regardless, better than being unarmed. When properly trained, one of the foremost lessons learned is to be aware of downrange. Not every teacher is interested, and if not, I would not want them to be. However, I would not be surprised that the most capable are in places where least likely to be called upon, as such communities tend to be more gun-friendly and thus more attuned to knowing how to handle firearms.

My feelings/thoughts on this question is nobody truly knows because each situation is different. However, taking down an active shooter is not the same as taking down a trained Marine/soldier in combat, I believe in a school shooting, a teacher armed is better than not being armed. Any chance to defend yourself is better than no chance and the knowledge that teachers/staff are armed will reduce the chance of a "active shooter" taking place in the first place.

study what the Israeilies have done to harden schools by arming and training teachers.

We are the blame for the way people think about these school shooting. 40 years ago people cared about each other and more compassion. Today they don't care who gets hurt no morality left in the world. Something has to give. People help make this world better.

First off remember I am now a FAG (Former Action Guy) with 45+ years of carrying a gun due to bad people with 25+ years as a LEO State and Federal. Most of the school shootings have been done by amateurs, not pipe hitters. ANY resistance to them may have made a MASSIVE difference. For them to have take gunfire regardless of how inaccurate would have possibly saved many lives.

The answer to your question is yes, if the teacher is properly trained. It's been working in Israel since the early 1970s. The PLO terrorists were attacking schools (aka soft targets) with AKs (the fully automatic kind) and hand grenades. The Israeli government put out a call for volunteers among the teachers who would be willing to be trained and to carry a handgun. The terrorist attacks on schools stopped when the terrorists were ending up dead with only minimal destruction caused.

Here's a sad truth about today's law enforcement agencies. They often spend more time training the officers about how to avoid being sued than training them on how to avoid being killed. I offered to train one guy who wanted to join the NYPD on use of a handgun. His response was that he didn't like guns and the NYPD would teach him all that he needed to know. Does that sound like an individual that you would want covering your six? Look at the idiot in Cleveland that killed Tamir Rice. He was dismissed from another department because he had a panic attack on the pistol range because he was do afraid of firearms.

I’d you were a student of that teacher, would you rather your teacher be armed with a ruler?

Hand guns are better suited for CQB in a building than long guns. Just because you have one type of weapon that is for a specific use, doesnt mean it works well inside a room.
Plus, the reason a teacher having a gun works to deter these horrible events is that it is a DETERRENT because you know someone has a gun, but not where they are, and it is a tactical disadvantage to the shooter. As we have had proven many times, shooters stay away from targets that have a higher likelihood of resistance.

Amazing how well people think LEOs are trained. Average officer only receives basic firearms training and only shoot on qualifacation day. New officers training may say the officer gets 40 to 80 hours but you have to remember that is how long the class is. A good part of this is waiting for other people to shoot.