I allow students to work in groups when we write songs and always give them guidelines for what I expect. If it is about a historical topic (such as the plague) I give a minimum number of facts required. If the song is about something procedural (such as "how a bill becomes a law") they must cover all steps of the process. Students are also given time requirements so they know how long they have to work on their songs. It is important to include time for "rehearsing" if you plan to perform them.
I make sure to use performance opportunities not only to teach content but also "good audience" skills. Students should be supportive of their peers and have fun. I HIGHLY recommend recording the songs! Not only do the kids like it but it is also great for sharing with parents.
Here are some songs from the 2009-2010 and 2010-2011 school years.
Songs on the Black Plague: