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

Good points by you and SM... As an old guy VN vet, I've watched our culture change and sadly agree that not only is it a difficult situation in the active shooter confrontation (though I think my WW2 vet teachers would have been able to cope better than today) but the whole nature of our society is so changed that those who have the 'know how' and respect for guns have become far outnumbered by those who 'just don't get it'.

First, we should put an active law enforcement officer in every school. That is the right way to protect our kids.
Second, we should put an appropriately trained and licensed school counselor in every school. This will minimize the mental problems that lead to kids becoming shooters.
Third, I am retired army with plenty of experience with a 9mm and an M16. I am currently a high school teacher. I have a concealed carry permit and I go to the range at least once a month. I am qualified to carry a weapon to protect my students and myself. Using a .40 cal., I can, and will if necessary, take out an untrained psychopath with an AR-15, no problem.

I agree with you

The proposed arming of teachers was never suggested as blanket commission or as a command. Only those who undergo training and evaluation would be armed. They don't need complete police training (as was often stated as an objection) because that involves the understanding and implementation of a lot of national and local policies regarding due process and legal precedent, and a more subtle, graduated response is necessary in day-to-day policing. No understanding of due process is necessary in order to see some nut firing a weapon in the hallways of a school- especially a student known by many of them- and immediately engaging them Some level of tactical training would obviously be necessary. As for the safety officer present in Florida, you do realize that he did not respond as he was supposed to respond. In fact, it appears all the Sheriff's Deputies who responded that day acted in either a confused or cowardly manner.
The shooting in Dallas and in Las Vegas are not the same as the school shooting environments. In one case, the sniper had the advantage of virtual concealment through confusion, and a large area where he could have been firing from, requiring a time-consuming evaluation of the tactical situation. You can't 'run to the sound of the guns' if the sound is echoing off multiple buildings and objects in the are. In Vegas the fact that it was a large amount of shooting and into a large crowd, with the resultant confusion among thousands of people, which made it far more difficult to respond. The shooting in Dallas involved a sniper as well- but someone who had the additional advantages of cover and concealment and also a superior tactical overview of the situation. Everyday police are never on normal duty prepared for a sniper to engage them. Again, none of the comparisons between a sniping incident at medium to long range and a school shooting are valid. They are not comparable situations. The Dallas sniper came armed for his 'mission' and he came armed for a different mission than that of the police being targeted. Again the police disadvantage was not caused so much by the difference in the weapons themselves, but by the entire overall tactical situation. That scenario is also not applicable to these school shootings, where the shooters seem to wander the halls aimlessly seeking targets of opportunity.
In a more confined space where their movement is more random, like a school building, the long rifle does not give the shooter a specific advantage over a trained and competent opponent armed with a handgun. That is especially true if that opponent is devoted to his mission of protecting the innocent and has demonstrated competency with their firearm. Any teacher who has the opportunity and presence of mind to intentionally use their body as a shield would have the presence of mind necessary to engage the shooter if they were armed, and so far every school shooting had teachers who did that very thing. Accidental shooting of an innocent could, sadly, occur when engaging the shooter. However, that is still likely to result in a more desirable end to the situation than allowing a shooter to wander the hallways of a school unopposed. In some locales, it could be 15 or 20 minutes based on local police response times. The concept of arming some carefully selected teachers or other school staff should not be rejected on the basis of a potential difference in weaponry, or rejected on any basis given so far by those who object to it.

I think a misconception is that teachers are all civilians with no experience or training with firearms. On the contrary, many teachers are also Reserve and National Guard, former police, corrections or other LEO's, former FBI, CIA, security experts, sportsmen/women, hunters, etc., people with a background of familiarization in firearms. When New York passed an public access Automatic External Defibrillator (AED) law, requiring all public facilities, including schools, to carry AED's they did not also require training! The law said that during all school functions, before and after school, trained school personnel must be on the premises ready to perform CPR/AED rescue! What did we do? We offered FREE training and then reminded the school staff, teachers, administrators, secretaries, custodia,maintenance and grounds staff, that if they took the training and agreed to be available while on duty to assist in an emergency, they could also use the training with their own family and loved ones anywhere at any time. We got the volunteers, trained them, and then assigned them to response teams based on their most likely location during their working hours and then assigned other to be available during after school activities that weren't already covered by coaches, who by law must already be trained in First Aid, CPR/AED's. With school security you could do something similar. Ask for volunteers with a background in the use of firearms, train them in armed active shooter response, for free, and then group them in to QRF (Quick Reaction Force) teams. That's how a teacher might be useful in an active shooter response.

I believe that no teacher should ever have a gun on them, they are teachers and they get paid to teach. Lock the damn schools, post a packing guard at the doors and install metal detectors or something but don't have teachers carry. What if one day in class they get to the point where some kids are just driving them crazy and they have had it and decide to use that gun then on them kids, lets say a teacher is dealing with marital or financial issues and they go off in class and shoot up the place ? what are we going to do then ? Blame the kids ?? Blame the gun ?? Blame the teacher ?? Put roving guards, door guards, gun powder sniffing dogs in these schools, raise gun ownership age to 25 , a 21yr old to me is still not mature enough these days to carry a gun !! Their brains are still developing !! Lets invest in our children's safety but don't have teachers carry also.

What does being a teacher have to do with it? Aside from taking and passing the CCWP class, he/she would have to be trained in its use and practice. Twice a year isn't enough to be effective for most people who've never fired a gun.
Part of this money that's supposed to be for educating and training teachers will include this. It's voluntary. Aside from hitting targets, they would have to know how to handle these situations.

Maybe or maybe not. I wouldn’t necessarily expect a teacher to offensively engage a shooter, however, I think that it would be better for them to be armed and possibly be able to defend themselves and the class if the shooter became an imminent threat, (let’s say that they were confined in a classroom where the shooter broke in), at least they might have a chance to neutralize the threat.

Yes. It's called training. Confidence. Proficiency.

Same as we are expected to fight against an enemy dressed a a civilian. Practice and the necessity to survive.

Besides. 1 shooter (example) against 10-20 teachers.

Is the enemy, foreign or domestic, bombing police stations and Military posts and other facilities that have armed guards and plenty of ammo to fight back? Or are they bombing public locations with the possibility of someone having a weapon is extremely slim? If you increase the presence of defensive forces that should hopefully reduce the occurrence of such terrible and hostile acts.

I believe they could and would with proper training. It is not for every teacher but for those who already gun enthusiast who practice and are willing to go through the necessary training I say why not. By all means I believe having former military and police in the hallways are part of the answer. At my church we have armed security and these are church members willing to do the training to protect our brothers and sisters should anyone come to do us harm. This begins with security on the outside and progresses to the inside and being able to lock down the sanctuary in an instance.