Have the teachers come stand in a line at the front of the primary, facing the children. Have them sing either one line or the whole song. The primary children then sing the same thing. Have the pianist, secretary, or other adult be the "judge". Whichever group sings the best gets to pick one person from the other team to come join "their side". (however, there's obviously no running-the chosen person is automatically on the team when they are chosen). You can make it more challenging by saying, "you can bring someone with you to the new team if you tell me what the last line in the song is, or tell me what comes after __, etc." I"m sure there are a billion ways to use this game.
The purpose is to have fun while singing any given song as many times as possible while learning the words and tune.