If I was you, I wouldn't go against the mother's wishes and be sneaky behind her back. If you have a problem, go talk to her. Don't take your frustration on the child. I don't think I like it too much if the school went against my mother's wishes and send me to a deaf school if she feels strongly against it. I would be one confused child.
A child's behavior only get worst when people teach him to not trust or listen to his mother (except for child abuse)
This doesn't mean you are not allow to use sign languages around her child because it is your only way to communicate. The child will pick up on that without you telling her that her mother is wrong.
