If it gets filled up before you're done, you could have a judge empty it with an eraser, depending on how well they sing. Or just erase the whole thing quick and start over.
I pick a new judge for each song. I look for a class that's singing their best and choose their teacher to judge, or I ask that teacher to choose someone from their class to judge.
This idea could work with any holiday or topic: Fill-Up the CTR Shield, Fill-Up the Easter Egg, Fill-Up the Temple, etc.