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

Of course. Two rounds Center mass fired from cover or concealment would do the trick.

Just read the one common denominator between all the school shootings is that the shooters
were fatherless. Now there was a link between aggressive behavior in adolescent male elephants and not having a male bull role model in Africa because they were hunted. Same for humans. When their testosterone is out of control they need that male to put him in their place

More to the point, is it realistic to believe that a teacher with no means of defense of any kind has any chance at all against an active murderer using any kind of weapon?

If teachers are armed, they will not be tasked with "hunting down" an active shooter, only with remaining in their classroom as a last defense against an intrusion by an active murderer. With proper training, and not very much of that even, a teacher who is mentally ready to protect her students at any cost will be able to bring effective fire on a murderer entering her classroom. Point at the door and press the trigger. Most people can hit a human target at 15 feet or less even under stressful conditions if the target is standing in a doorway.

Yup, you can do that....It only involves Training, Training, Training, Training and some more Training. It better be reflexive or your dead!

It only takes one well placed shot to stop a threat!

It depends on how much firearm experience he/she has. Since this is elective then adding situational awareness and tactical movement training would and should be included. I learned the basics fairly quickly in Basic and got better during AIT and on.
These shooters target gun-free zones and places where unarmed security guards are employed. Get rid of those signs while placing some plain clothes armed guards, as well as trained teachers and these instances, will reduce. There has to be a few people (depending on school size) that are on guard. Not chilling in some security room looking at cameras. They could look like they're doing something else while roaming the grounds.
Money will have to be devoted to this if it's going to be effective. I know there is a large number of Veterans who would gladly take up this job, myself included.

Training for active shooter is a good thing but real drills needs to be a real excercise not just a training document

Hitler had guns registered for Austrian’s safety. Then he banned the firearms for their safety. Then he invaded a defenseless country to occupy.

The framers of our Constitution, wishing only to persuade their fellow Americans to ratify the document, reluctantly guaranteed the several states the right to form and fund their own armies. The Second Amendment speaks to precisely that issue. It was never intended to arm individuals, just citizen militias. The recent decision by the SCOTUS to reinterpret that particular sentence as extending the privilege to individuals is particularly disturbing in light of these mass killings--by individuals armed with weapons designed for use by armies. People who have neither the training nor the reason to use these weapons as their creators intended are instead murdering their neighbors or obliterating wildlife indiscriminately.

I have to agree with Timothy's assessment of this situation. As a retired public school teacher, I shudder to think what sort of chaos would have ensued had one of my colleagues, untrained with any sort of weaponry, attempted to restore order with a firearm. As he pointed out, even trained police officers and soldiers make mistakes when exchanging fire with an armed force. They have training to fall back on when they don't know what to do. What has a civilian teacher, whose only military experience may consist of serving as a personnel specialist in an office, to fall back on in a crisis? Not a darn thing. Such a person is more likely to accidentally shoot a student than to stop a shooter who has infiltrated the campus. Rather than arm teachers, who already are called upon to do too much, we should place security personnel who have been trained in police officer standards and weaponry on school campuses. That way, the teachers can teach the students and the guards can keep the teachers and their students safe.

Less than 3% of ALL shootings are with a long gun, much less an AR15. How many school shooters have used an AR15, or semi automatic long gun (same thing without a model number) for that matter?

School shootings invariably are "danger close", with the shooter in the school, even in a hallway, easily within range of any semi-competent person armed with a pistol.

There has been NO proposal to arm ALL teachers or staff. For one thing, it's not needed. Another thing is that not all teachers would want to be armed, and that's OK, it's their right.

One problem with police tactics in these circumstances is the policy of the officer waiting for a back up, or the swat team. Once confronted, most of the school shooters commit suicide. If one of the deputies had entered the school and confronted the shooter, how many less would have died? I for one would have a difficult time living with myself if I sat outside hiding under the circumstances.

PV2 Case we're all military here, and know there's a cost. If the one child died and 16 (or more?) were saved, was it worth it? (I understand, not to the parents of the one.)

The reality is, 98% of all mass shootings happen in gun free zones. Why, because the victims can't defend themselves, are sitting ducks and easy to kill a large number. Just having armed staff in a school is going to dissuade some of these shooters.

Finally, I too an tired of the implement getting blamed instead of the person, and ultimately society. We don't (usually) blame cars for killing people, spoons for making people fat, or ban hammers for killing people. (More murders by hammers than rifles.) You want to take God out of schools and lives, deny absolute truth so that anything goes, promote violent video games to our children, and make sure to start them young, coddle our children/young people with no consequences, everybody gets the prize, eventually you're going to get what we have now. Our society, backed by the progressive movement created this, and they're not about to admit their agenda has failed this country.

The 2nd amendment is not about hunting, it's about self defense, both against bad people and tyrannical government. It's the first step of tyranny to take them away.