I'm pretty confident Donald is talking about armed teachers just as a distraction. When he has things he wants to get done, he says stuff like that to keep the media busy. It aways works.
But as a person who is generally pro-gun rights but also has teaching experience, arming teachers doesn't make sense from any perspective. As a teacher, a lot of technique goes into maintaining control of the classroom without students knowing you're maintaining control. Not only does this make stufents feel at ease enough to learn properly, but it also keeps students from challenging your authority if they don't know you're exercising it.
But it actually makes far less sense from a gun safety standpoint. A classroom is not a secure area. Kids are frequently behind the teacher or out of view. Like with police, anytime a student attacks a teacher or is fooling around with a teacher's desk, you are no essentially in a gun fight since accessing a key to the guns would lead to that. If a student is fooling around at my desk, I don't want to treat that student as if we're in a gun fight.
Every lesson would feel like a traffic hot stop. Needless to say, no one would learn anything and some kids would get KOd and some teachers would go to prison.