What material most closely resembles/emulates dog's excrements?
You'll never be able to get to 2-3m accuracy with this. With a reproducible sample, you could make a few dozen tests and create a convincing statistics.

But dog excrements? Even if they always used the same dog, which they kept on a constant diet, and which you had impounded, it'd be impossible. The sample is much, much too complex, and, unfortunately, very much alive. (I don't mean the dog.)

I say stick to old-school policework: Keep interrogating your pranksters until one of them sings. ;-)