Should teachers have guns? Only if they feel comfortable with them. There is no point in having Mr. John or Ms. Suzy fumbling with their .38 snub-nose when the person comes in. There should be no pressure for anyone, plus a strenuous test for them to be able to. Basically, they should be able to conduct a gun safety course (properly) to do so.
The weapon should be concealed and unloaded. Yes, unloaded. My suggestion would be to have the concealment be in the form of an ankle holster or even in a locked drawer.
The ammunition should be either separately locked in a separate drawer and not obvious.
The concealment is necessary to help the students not know who is packing a weapon and who is not.
Now, if teachers are going to carry guns, they should also be allowed to restrain people, students included, with little liability or impunity (obvious issues notwithstanding). Students used to fear teachers and faculty, when necessary. Not think they can run over them. The vast majority of teachers I spent time with were nice, cordial, and very helpful, but if you stepped out of line, you were expecting all hell to break loose. Now all children fear from teachers are, "Please, Suzy/Johnny, stop that." Zero recourse today. Hell, some schools are doing away with punishments and detentions. And we wonder why we think some kids think they own the schools. In their minds, they do- and we let them think that.