Errr, why wouldn't any teacher already be carrying their own personal hand gun anyway?...they are allowed to under the 2nd amendment...so there probably already are "guns in the classroom".
What Trump floated was a consideration to properly train some school teachers in the use of a hand gun in case some lunatic decides to start shooting, at least there would be a chance of taking the idiot out before too many innocent lives were wasted.
The idea has some merits being I doubt they will ever have their Bill of Rights amended to remove the individual's right to bear arms.
Unfortunately I don't think they will never stop some loon stealing a gun or "borrowing" a gun to go on a shooting spree.
