You can make the class fun while still disciplining them for disruptions.
I disagree that Jiu-jitsu and martial arts in general are skills of little value. The skill itself maybe be used little if ever in a real world application, but I guarantee you that if you ever have to use Jiu-Jitsu in a real world application you won't call it a skill of little value.
A fire extinguisher may sit in your house never to be used but you wouldn't say it's a tool of little value.
Second Jiu-Jitsu has many benefits besides self defense including increased confidence, problem solving skills, learning to deal with and overcome extreme discomfort and adversity, social skills, discipline, respect, and probably the most important lesson of Jiu-Jitsu, checking your own ego and learning humility.
Humility is something that many people don't know anything about and Jiu-JiJiu is a great teacher of that.
Obviously you can learn these things doing other activities as well, but the personal nature of the sport really hammers a lot of these lessons home.