Nolan Gould (Luke Dunphy) is actually very talented at the banjo in real life. He is seen playing the banjo in this episode.
In the scenes before and during the Faculty Follies, the Grease chorus functions as a Greek chorus would in a classical drama, narrating on the ongoing conflict between Cam and Señor Kaplan. This is another play on words employed by Modern Family writers.
When Cam attacks the other teacher and runs on stage, you see somebody else