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

The big issue is deterrence. Schools are a soft target where normally only the School Resource Officer (SRO) is armed. If teachers are allowed (not mandated) to be armed, the school becomes less of a soft objective. How often do mass shootings happen where ther is the possibility that someone there will be armed? (Hint: not very often.) As a teacher and a veteran, I would gladly be armed while I teach. I predict that I would probably never have to unholster my weapon, simply because it is there and visible. (No, I wouldn't wear it on my hip.) When the word gets around that some of the teachers (the well qualified and approved ones) are armed, the opportunists will choose another venue.
I truly don't like my chances of taking on a shooter who has an AK-47 or AR-15 while I am only armed with a 9mm pistol. I do like those odds a lot better than me taking on someone armed with and AK-47 or AR-15 while I am armed with a textbook, shoe, fire extinguisher, or desk.

any armed response is better than cowering in a corner and waiting to be executed. Every day there are thousands of civilians who use weapons to defend themselves successfully. No one knows how they might react in a live fire situation until they are in it and that includes military. In combat I've seen men who I thought would be cool under fire freeze up and men I thought were the weak ones turned out to be good under fire and an asset. One guy in particular who was always the tough talker actually deserted when we got orders to go to Iraq and was last found hiding out with greenpeace.

Furthermore, some teachers are veterans so don't discount their training. Lastly, after leaving the army I worked in code enforcement and was around police a lot. They don't train nearly as much as some civilians. My neighbor is a United pilot AND a competitive long distance shooter. I would give him a MUCH better chance of taking out a bad guy than any regular soldier with no combat experience OR any cop. Contrary to what you see on TV, regular cops don't actually train that much which is why they had a bit of difficulty in taking down the guy in Dallas.

This question is silly and illogical. The answers to any possible question on this train of thought are not even debatable.

Is it realistic that a person could stop a murdering shooter with a handgun? Yes.

Is it more likely than doing it with their bare hands? Yes.

Is it more likely that the teacher will accidentally friendly fire on students more often than the murdering shooter will? No it isn't.

Equal or greater force is the only way to stop someone with the intent and means to kill. You might as well be sitting here arguing that our infantrymen need to go into firefights with baseball bats.

Absolutely....

No corporal but they DO NOT allow anyone with PTSD to hang around schools in makeshift OPs,so it's the BEST IDEA we have...other than reverting to JSOC grads only ,as school instructors.
I suggest the application of FATHERS WITH SPINES instead.

way to many criminals walking around looking to make a point or name for them selves. Ever noticed how in the old days very few horse thieves stole a second hors

Those who raise fears of the Police shooting teachers are insulting the intelligence of the First Responders. Look at ANY photograph of the offenders in these incidents and compare them to a picture of a Faculty Member. I am Chief of Security at our Church. Those authorized to carry a firearm at the Church are required to carry (and display around their neck, when needed) a fluorescent Identification Card. A copy of this card is on file with the local Police and Sheriff's Departments. This is in addition to them attending a training course given by our local Police Department, qualifying on their firearms, and participating in a Laser Scenario Deadly Force Training Course. Additionally, they are taught that they do not have a duty to neutralize the Threat (as Law Enforcement does), they have a responsibility to protect Innocent Lives - there is a difference. Using emotion and fear to paralyze a person from action is an old tactic used by those who cannot stand up to a logical argument.

Being shot at and seeing a clump of dirt fly up next to you is one of the scariest situations that a person can experience. Your (my) immediate response it to assess your armament, defenses, and choose your course of action. If you have NO weapon, your course of action is highly limited, and your ability to gain any control is virtually nil. I carried a handgun as a law enforcement officer for 22 years. It is the firearm that you take, when you do NOT intend to be in a gunfight. It is not the best option, but it beats the snot out of teeth & fingernails.

Absolutely not. In fact I believe an armed teacher is more of a liability. And they could be identified as a target themselves.

Question #1, Could they respond correctly, and engage the target?
Question #2, What if the target is a six year old kid? Could they use lethal force?
Question #3, Could they even use deadly force?

After seeing 30 years of “The troubles” in Northern Ireland these questioned must be answered before you even see a weapon.

During the Iranian embassy siege the SAS engaged anyone carrying a weapon.

You assume every shooter has an AR15. You are also assuming every teacher has never held a handgun, and has no idea how to use a firearm. The arming of teachers was mentioned and was never going to be mandatory, but voluntary, AFTER training.