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

Realistic; but not a certain thing. The hope is that the murderer will not expect armed resistance and would in effect be setting himself up for a hit by the defender.

Realistic? Not really. Better than nothing? Absolutely. A shooter is more than likely going to head to the path of least resistance. Even a cap gun might make a shooter look elsewhere.

It's better than fingernails and harsh language. And these teachers aren't going to be hunting the active shooter. They'll be in lockdown and defending their students with that handgun.

the cop was a pussy. Let the teachers decide. Better to die fighting, than to die cowering and begging. Fight back. Most teachers would fight to protect the kids.

Most of the Active Shooters in recent past have had little combat type experience or training. So a teacher with a hand gun, and a minimum amount of defense training could be the shield all the soft targets need in a school building.
We as a society, like many previous commenters have stated, are to blame for the violence we are experiencing. We have fundamental changed our moral compass as a nation. The left leaning politics and segment of our society that is avoiding accountability, in search of freedom from perceived religious persecution (all while those that would question their behavior have become more tolerant); in order to achieve that freedom they have to remove personal rights that are guaranteed by the U.S. Constitution.
Our society has more men and women that were raised not by their parents, but by sitters or daycare. With less emotional attachments and declining membership in churches, we have generations of tolerance, that let gray area offenses slip by without accountability. Willing to pencil whip required reporting, and a loss of productivity due to a reduced work ethic. Our society is slipping as we speak!

Wee said sir.

Why army teachers ? Just hiring more security In the schools

The question itself is unrealistic. So many belief systems among so many people. So many factors to consider. This question is just another invitation to an argument.

I taught school after 24 years in the US Army. My opinion for what it is worth is not many teachers could be trained quickly enough to be a person responsible for removing the threat. My church made the decision to arm certain persons to carry weapons. There was a local police officer invited to the meeting. I asked him if he had ever taken a human life. He answered no. I then asked him if he could and he replied "I don'f know." I then asked the members designated to carry a weapon the same questions. A few of the designated immediately replied, "I am out of here." I then turned to the Pastor and board members and asked the same questions. You could have heard a pin drop in the church. The Pastor asked me if I thought I could take a human life. I replied that I had taken many human lives while serving in Vietnam. He and the local policeman both asked me to explain what happens to the person that takes a life. I looked at every person in the meeting and replied, "You don't want to know." I think teachers would have the same reaction as the church members.