The idea of singing to an instrumental background was fine. The problem was that when you're singing along in a large group, the interior members are unable to hear the music. There are usually a greater number of interior singers than there are along the perimeter, so the interior singers will dominate and set the pace.
The group needed speaker monitors placed nearby (or earpieces, but those are a bit too elaborate for a middle-school chorus) to allow them to hear the music in order to maintain timing. Otherwise, you get a cluster-f**k like what Wednesday's audience witnessed.