On-task vs. Off-task Student Behavior
When students are actively participating in the rehearsal through playing, singing, moving, listening, etc. students are on-task. When students are not paying attention, talking (unrelated to classwork), or disrupting other students they are off-task.
It is possible to run a stop watch on a class. You will find that in music classes where the teacher spends a lot of time talking, the students will often have behavior problems, morale problems, and the dropout rate will be high, because the students spend much of their time off-task.
I once observed a student teacher and kept track of how much time the students spent playing their instruments. During the first 20 minutes of the class, the students spent a total of 2 minutes playing! Of course, the student teacher was very upset after rehearsal, complaining about the students' behavior. I explained to her that the reason they are being so disruptive is because they are off-task and become bored and distracted when they are not actively participating in the rehearsal. I asked her a simple question: "How much time can you expect an 8th grader to sit still and do nothing with an instrument in their hand before they start to become bored, distracted, and disruptive?" She answered, "2 minutes." I then told her she should plan her rehearsals so that nobody stops playing for more than two minutes.
In the above example, we know most of the students spent about 10% of the time on-task. The rest of the time we simply don't know if they were on-task or not. We don't know if they were listening to what the teacher was saying.
Having all students on-task 100% of the time is the goal. If a teacher can keep ALL their students on-task for at least 85% of the class, the class will generally be effective and there will be few classroom management problems.