After learning that Emma loved The Rocky Horror Picture Show, Will decided to make the group put on that musical for the school.
His students were totally into it, with the exception of Finn, who was unsure about being on stage in his underwear because he was oddly insecure for such a jock. But Rachel helped him overcome those fears a bit and the practices seemed to be going very well.
However, Will later took exception to Carl being included in the play. He actually inserted himself into a role in order to keep up with Emma's man and asked his crush to rehearse a scene with him that involved her character trying to seduce his. It was hot stuff.
Soon, though, Sue would be the one to talk sense into Will. She had agreed to do a report on the show in order to show parents what sort of inappropriate arts their kids were being taught at school. Will discovered this plan (thanks to Becky), confronted Sue over it and got an inspired talk from the coach. She was on to Will, she knew he only put on the play to attract Emma - and Will realized he was crossing a line. He was pushing boundaries and asking his kids to stage a lewd performance just because he wanted to steal another man's woman.
In the end, Will told the kids this, apologized to Emma and said they could still put on the show, but only for themselves. They performed a final number in an empty auditorium.