Your question is simple. Not all gun owners will be interacting in our Tennessee Schools, with our students on a daily basis, so 40 hours a year is an unrealistic, unnecessary standard, simply meant to disarm the vast majority of weapons owners, and weapons carriers, including Carry permit holders, who have passed state approved training, regarding safety, use, legal aspects, weapons handling, firing familiarization, range instruction, qualification, state, local and federal background checks, carried out by the Department of Homeland defense.
I am not saying this is a good idea, in our schools, at least not for the vast majority to teachers and employees, but the standard if applied to everyday Tennessean or citizens of other states, would simply be an unconstitutional attempt to disarm US Citizens, and simply will not fly.