Dick Oakes advises: do not try to teach your neighbor when someone else is teaching the class.
The Miami Folk Dancers exhort: there should be only one instructor at a time. Do not try to teach when someone else is teaching.
The Narodni Folk Dancers echo the sentiment: please keep quiet so that the teacher can be heard by all.
This is a great rule of thumb across various styles of dancing -- folk, contra, square dancing... anywhere there is a caller, cuer or teacher. It's disrespectful to the teacher, as well as to fellow dancers. If you already know the dance, model good listening for newer dancers. You can transfer so much knowledge through joined hands without saying a word.