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

After a series of fatal school shootings by Arab terrorists the Israeli Government mandated that ALL teachers would be trained and armed. That was in 1994 and there have been two shootings at schools in Israel since, with ONLY the terrorists dying, by being shot by teachers!!!

Well, it depends on the teacher.

A lot more realistic than if they don't have a weapon. When my daughter was in junior high, the principal was a reserve Colonel who had been active, including Vietnam. I have no doubt that there was not one day my daughter was in that school, that she was not protected. No doubt either that the Colonel and his henchmen were armed. I was never told that, but I know the man, and saw concealed carry that slipped on more than one occasion on his henchmen. I doubt if someone not of my background would have spotted it, I was a deputy sheriff at the time. By henchmen, I mean at least one retired senior enlisted and several former enlisted. They were the custodial types.

When my daughter went to the senior high, I was a little more worried. Henhouse just waiting for a weasel. Victim disarmament zone.

My point? Give school security to retired military types. Make it a condition of hiring. Much better than walmart greeter....and that would solve your issue.

And, while choice of tools matters, especially past 50 yards, one is fighting the other person, not his attachments.

Oh, and by "henchmen", just in case a civilian reads this and doesn't understand, I mean the Commander's (in this case I guess the principal) most trusted subordinates, and that would be senior NCO's. I was a henchman, I am proud of he fact that I was a good one. Not to be confused with "minions" who are a lower class of henchmen. A good minion may become a henchman if he does it long enough and is good at it.

Absolutely, Yes.
In addition to the deterrent value-- which is huge by itself-- active shooters are not 'good shots'-- especially the mentally disturbed children we see in school shootings. Not to belittle anyone's loss or pain, but this a**hole unloaded for 6 minutes in a crowded school hallway and ONLY 17 people were killed? That is 17 too many, but how many would it have been if the a**hole actually had a clue how to use a gun?

Taking on an automatic weapon with a handgun is wholly dependent upon 3 things: confidence, ability, and position.
Confidence-- you have to not crack under the 'fog of war'. Know your capability and rely on your training.
Ability-- You NEED to hit what you are aiming at. We do not need a bunch of bad-shots creating 'friendly fire'.
Position-- Get to where you can be effective before you make a move, or you just threw your life away and gave the shooter another gun. If I'm a teacher in a classroom, I'm not going to jump out in front of a shooter who has an automatic weapon and try to take him out with a pistol. I'm going to lie in wait with the front sight post chest-high on the door and wait until he opens it to get the first shot-off.
If he is in a hallway, I come up behind him and shoot him in the back (either from behind the door of a room he passed by or from around a corner). Active shooters do NOT cover their sixes.

Position being a key part of this-- the more armed 'good guys' there are the better. It increases the chances that at least one of you is going to be in an advantageous position at any given time which would minimize the potential loss of innocent life that would ensue while the 'good guy' maneuvered to get into a position to take a shot.

All that said: I've got a lifetime of military experience which started in the military police. I trust myself in this situation. I'm NOT suggesting that every teacher should try to gain the level of training and firearms experience necessary to be effective in these situations, but there are a lot of veterans, military retirees, former police officers, etc. who ALREADY HAVE the training and experience who are only lacking the authorization.

Certainly not an untrained teacher, but perhaps if they want to be. On the other hand, I'd rather find a process to prevent it in the first place (easier said than done). I saw this article this week, and it's from a veteran on this very subject. You all can say his experience is accurate much more than I can. https://www.charlottefive.com/arming-teachers/

L CPL Timothy McCain — Something needs to give. We either hire or not, armed guards to protect our children. There was so many failures during this shooting. Next they going to say the gun was the bad guy. The gun only did what the shooter ask it to do. I would probably get my tail shot off but I would like to think I would do something.

Yes, it is realistic. In close quarters combat a rifle is not always an advantage. They have a longer turn radius, they can bump into things and make it harder to get on target etc. Their primary advantage is range which is somewhat nullified by being inside a building with lots of walls. With a semiautomatic handgun you can lay down a fair amount of fire and you can maneuver a little easier too.

Most of the school shooters are not trained in close quarters combat. Most of the more infamous school shootings (Columbine, Sandy Hook, now Parkland) were done by Students or someone pretty young and no real experience.

Your Dallas example, which was outdoors, and the cops that were killed were hit in the initial surprise attack by someone that had taken up a position that a rifle gave a very significant advantage to. That is not normally the case in school shootings.

You will have to accept the fact that in any live shooter incident there will be confusion, panic, chaos etc. I would still rather have a few teachers (who are willing and trained) to have access to a firearm to offer up a defense. The bottom line is that right now, for the shooter, there is almost nothing to stop him. Even the cop on campus in this case failed to do his job and was suspended (see my post if you have missed this part of the story).

There was a so called police officer there, cowering like a scared rabbit.

Across a football field? Not likely, but possible. Inside a library or cafeteria? Very possible. As he enters a classroom through the door? Yeah, pretty easy.

Of course. I believe they come from the same species of mammal that soldier emerge from: homo sapien. Some might have little apptitude and talent and other may have it. Arm those capable. Keep the number of armed teachers private and make carry concealed and that fact alone could be a deterrent.