I don't think it's a good idea either, cuz ALL the kids know who that SRO is, and he could easily be ambushed. Now, IF all the teachers or the ones who wanted them had guns, the kids would be MUCH safer.
Also that person is going to go years, maybe decades, maybe his whole career without an attack. It was clear that at least one of the school shootings that the SRO or whatever he was called was unprepared for an actual attack.
Obviously these need to be collateral duties for the teacher since they are so rarely called into service. That would require a huge attitudinal shift in the Schools of Education.