Have you never worked somewhere that you are told you have the authority to make decisions, but as soon as the "client/customer" complains to the supervisor, suddenly accommodations are made and your decisions don't matter?

Teachers can say no. But then parents complain to principals, and then principals tell the teacher "you need to help this kid succeed in school". Or parents complain to district admin, who calls the principal and tells them that they need to help the kid succeed.

My daughter was told she was not allowed to fail kids. It was up to her to work with them and help them pass the class. The fact that she'd been begging the kid to do work all semester was irrelevant. Suddenly she has extra work to help the kid get a passing grade.