When Ms. Dewitt asks Curtis if 'the sheep are on the roof,' it is a reference to a prank in which one would let four sheep out on the school roof and have numbers spray painted on them. The number are 1,2,4, and 5. When the administration of the school lets the sheep down, they'd be searching for days to find the 3rd sheep that doesn't really exist. Unfortunately, Big Fantastic was unable to get actual sheep for the episode.