I don't think it's that kind of note. I believe she's referring to the one they send home with everyone, asking permission in advance.
I wouldn't sign, either.
If my child had done something wrong and was about to receive a paddling for the misbehavior, I would be the last person to stand in the way. That's just who I am. It's the same as if he was stopped by a police officer, I wouldn't stand in the way of him suffering the consequences of his actions. Mommy doesn't make thing right as you grow up. You have to learn to respect authority figures. The first is mom and dad and the school is next. Then it is the law.
I do think if parents have some unusual complaint with a member of the administration of the school, that shouldn't alter the chain of command and authority the child should respect ion the school. Go in and talk to see what happened. Then if discipline is warranted, but oyu still feel you don't want "anyone to touch your child", fine. YOU paddle your child in the presence of the administer or teacher, so the student knows they didn't "get away" with Mom coming to the rescue. But the student MUST suffer consequence of the action and it must be as severe or more severe than the school's action. If you don't get ready for problems down the road.