Instead of teachers telling students "guns are bad", should they teach how to safely operate a firearm.
If we are talking public school teacher, I don't really think it is the teacher's job (or probably specialization of knowledge) to teach students how to use firearms. That's an outside of school thing. However, I also don't think it is a teacher's job to espouse opinions about their personal beliefs on a lot of issues especially in K--2 education. In a university setting especially, intelligent debate should be encouraged on a range of topics.