I'm not for a policy that mandates that teachers carry in the classroom. I believe that teachers teach and armed guards guard. But I think the states and their districts need to decide how they will protect their schools.
However, to answer your question, unless the schools become fortresses that no weapon or bomb can penetrate, and there are guaranteed measures to ensure that violent individuals don't enter them, guns, in the hands of people who train and carry to protect others reduce gun violence of malicious individuals the same way armed guards in federal buildings reduce it, or that a gunner's mate standing watch on a ship as it ports reduces it, or a bank security service reduces it.