If it continues to be a problem, you write/talk to the parent(s) and say, "I'm afraid [name] continues to be disruptive during rehearsals and it would be best if she did not continue participating in it for now. I would be happy to give her another chance in three months or so if she is willing to give it another shot, as long as she understands the expectations."
Eventually the others around here will shush her themselves as the entire group is waiting on her to shut up.
Catch her off guard -- that is, give a simple instruction which you are absolutely sure she would have heard, had she been paying attention, and ask all the choirmembers to implement [whatever it is]. When she can't, she'll either drop out or pipe down.
why do so many choir members look for attention in choirs?
I suspect no one pays much attention to them elsewhere.
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