no, but a large chunk of them are! Look, that, coupled with "zero tolerance" policies, astounding ignorance of applicable law, and personal political indoctrination, doesn't speak well for parents who have to fight with administrations to keep their kids out of unnecessary tangles with people who can tag the kids with reputations that follow them their entire academic career. A "hit", "read" or acknowledgement of scent by the dog is not necessarily probable cause to search, but what do you think administrators will do with that information? Nothing? I doubt it.
I'm glad you are employed in a school district that is 2A friendly. School districts like yours are fairly uncommon these days (although numbers are growing, which is good). Frankly, if a kid brings a gun ("unsanctioned") to one of your schools, it sounds like the problem can be solved with a phone call and a meeting to pick up the firearm. Good for you. Need more districts like yours.
But even the one I'm associated with, as conservative as it is, has immense influence over how kids are perceived by other adults, especially inside the schools themselves. Applying the dog in the way described in the OP could cause lots of unwanted issues with kids who are otherwise model students. And the dog has no idea about what makes "legal" or "illegal". Someone else gets to decide that. Who do you think gets that privilege?
Here's where I'm going to "get real". If you think the dog is going to "hit" on traces of "legal activities" as much as you say, then it sounds to me like that's going to make up the majority of his day or two at the campus. He's going to spend the majority of his time indicating on a school teacher who checked her oil that morning, or a kid whose folks took him deer hunting the day before, or the janitor who just used a solvent to clean up a crayon mess on a wall. If that's the case, why even bother with the dog in the first place? And why distract the student from being focused on learning?