The students know, but they're not talking. The dog knows, too, and is happy to show his handler where it is.
The schools all have the combinations to all of the lockers. You could find out what is in each locker without a dog. And without making kids put two and two together, when they learn about WW2 and how they used dogs to search for Jews.
Whats next walking those dogs near everyone's house? Obviously you see it as being a benign activity, do you feel the same way about stop and frisk?