Most likely your child has gotten to the stage in development where he or she has started to recognize his or her own self in comparison to the outside world. At this stage a child does a lot more passive learning in the form of observation. Please do not force your child to actively participate. If he or she wants to watch while taking in the musical activity then that’s the point which he or she is at in music development. Now if a child is neither passively or actively participating but rather gazing off or doing other activities then maybe there’s something else going on.
It is important to keep an open dialogue with the teacher about these things and through observation and discussion reach a conclusion about the child’s developmental stage.