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

That particular cop got fired for running away as far as I know. If I am wrong I apologize(nothing against police officers). The thing is our teachers should have a choice. If they choose to carry they should be afforded the training to use the weapon properly in coordination with LEO's, and it would be likely that more than 1 educator would have a weapon and the training to stop a shooter. Would I take a someone trained and armed with a handgun against the majority of individuals sporting a so call assault rifle? Hell Yes, because those educator's have a steak in the game. I would bet money that more than one educator would step up, in every school, so you then end up with a team of trained professionals against a derranged individual. I would love to be able to send my child to that school. It would also show our children another reason for our second amendment rights, so they can learn that guns are just tools and it is better that good people stand up to use those tools than have them be taken away and our children left to die

Been said before the reason this occurs is because we have already surrendered to much of our freedom to our government.
They now decide how we discipline our kids and how our kids are educated. That needs to change we need less government control of individual rights.
There are between 20-28 million highly trained , armed and motivated veterans.
That could be paid or volunteer at our schools till we get the government back in its box. I guarantee the number of incidents at schools would diminish.
And bad guys would find another soft target.

The Israeli commando raid (Operation Yonatan) on a French plane full of Israeli & foreign citizens held captive at Entebbe airport in Entebbe, Uganda on July 4, 1976 is a perfect reply to your question. Study it.

It may or may not be realistic as it depends on the individual's training and on the situation, however I think it would be better, if a teacher finds himself/herself barricaded in a classroom with a maniac roaming somewhere in the school, for that teacher to have a firearm rather than a bucket full of rocks to throw! Even if they stay barricaded in the classroom for the entire event, I would rather see that teacher armed.

Yep ! Especially if your Marine Corps trained. I've been out of the Marine Corps for 30 years and I was put to that very test last year. My nephew was getting married and his future wife's Family lives on 64 acre farm. This was an interracial marriage, her family were avid gun owners and hunters. My city slicker family was ( I Thought ) democratic anti- gun knuckleheads. I wouldn't give them a water gun, much less a real one. I say this because they all hated me when I came home from The Marine Corps with a few guns. My dad everyone no one wanted the guns in their house. Well to my surprise here in 2018, We all go out to his fiancee's house to go shooting I will silently praying that my family would shoot themselves in the foot. I brought allow my ruger P89, 15 and 1 in the chamber. I figure I will show my nephews future family that at least one of us knew what we were doing. When my brother and his sons in my cousins pulled up in their truck. I almost had a heart attack. All Of a sudden my family were the biggest gun collectors That I've ever seen ! They had modify AR fifteens they had Sig Sur 9 mm 40 calibers, They had Colt 45 calibers , Colt compact 45 and I think a Benelli shot gun. But all eyes were on me when I stepped up to the target, They wanted to see how a marine would react and how good I can keep my concentration my eye on the target when under fire. So as I readied myself and began to fire. One of my future inlaws, crept up behind me and unloaded a 30 round clip, to see if i would poop my pants. When i didn't flinch in the slightest, they gave me the thumbs up and walked away. I earned all their respect from that day forward. So yes, one can successfully defend against an idiot with a ar-15, you have to have to maintain a solid mentality under pressure.

I guess I am not seeing the downside of the idea. If the decision to proceed is based strictly on whether it might or might not work then the wrong criteria is being used.
While the gun toting teacher might make themselves a target for the shooter aren't they already? If you and I are armed teachers at a school while the shooter is shooting at me I hope you are taking a clear shot!

I think teachers could handle it if trained in a defensive manner. In other words do not train them to seek out the shooter or close on the shooter (that is the job of military and police). Train them to ambush the shooter from a defensive position. The key word here is Training. Most schools barely require their teachers to go to remedial training to "teach" so training them with the proper use of weapons and defensive tactics would be the problem.

All teachers are not trained, nor do they desire to be armed. However, if they are armed, trained, and choose to carry, why not let them use their own judgement as to whether to engage? Of course there is the risk of an accident, but I think the vast majority of the time that the outcome would be enhanced by having a teacher, or anyone, attempting stop the shooter.

If your child were be threatened, wouldn't you choose to have someone, anyone, trying to stop the threat? I believe that we have the right to carry, and with that right comes responsibilities. I feel safer knowing that people have the right to carry assuming they meet certain training criteria and are mentally stable. Only the person at the scene of an incident can decide that he/she is in a position to confront an AR-15 with a pistol or even a ball bat. I would accept the small possibility of an accidental shooting vs. no resistance at all to a potential shooter. Admittedly there are probably very few situations where confronting an AR-15 wielding bad guy with a pistol is advisable or responsible.

Yes, without a doubt!
The situation would be all close contact and a pistol is the preferred weapon!
Also, I’m no world class shooter, but I do practice on bowling balls at various distances out to 50yards with pistols!
So do my dad, my son, daughter-in-law, wife, daughter, and granddaughter! They don’t hit with every shot, but it’s great training!

I dont expect a teacher to hunt down an active shooter. But I would expect them to lock the door and try to secure the children in a closet or back of classroom. Then as last defense be last chance effort to save children.