Weeeell, technically it was Katara who realized they could be trixy with the fact that it didn't work...

Katara: Sokka, you're a genius!
Aang: ...did the definition of "genius" change in the last hundred years?
Sokka: It's all right, Aang, let her dream.
Katara: No, really! Sokka's plan didn't work...but it looks like it did!

Or something to that effect. *rewatched that episode last Tuesday with the neighbor kids, so dialogue is still fresh in the brainbox* Ah, the Water Tribe siblings. Faking Earthbending, faking Firebending, bullying hapless bending students...they work so well together.