Positive reinforcement for negative behavior

As I've moved around the past several years, inevitably, I'll play for primary. Right now I'm only doing it temporarily, as they wanted somebody decent playing for the primary program in October. (There's nobody called to it right now, so one poor unfortunate was playing the melody and that's it.) It seems that none of the primary children are ever taught how to behave, but today was even worse than usual.

The kids were acting up way more than usual today. Instead of trying to get them to behave during sharing time, they interupted the lesson to sing a "wiggle" song. Do you realize what you just taught them? You just taught them that if they act up in class, then they will be rewarded by being able to get up and move around to some song rather than participating in a lesson.