I think if teachers, principals and staff want to carry conceal then they should be able to under the provision they have received proper training. I wouldn't require anyone to carry a gun and I wouldn't put it in as a requirement to be hired. The NRA has already said they are more than willing to provide training.
Indiana law prohibits cc at schools, including preschools, unless that person is designated as part of the security team for that facility. I'm on staff at a church that has a preschool and after receiving my license to carry, asked our leadership for such a designation. They did so and I carry everyday. Under that law it would be fairly easy to go ahead with selecting and training teachers to carry. Of coure the biggest sticking point would be the school boards and administrators (and their lawyers). A great instructor I have used has said he would be more than happy to develop a program specifically for school personnel and work with them. I agree with the other comments that only those teachers/personnel who volunteer & pass proficiency should be allowed to carry. Let it be known publicly that there are armed, trained adults in every school, but their identities shouldn't be known even by the students.