That's exactly what the parent would have to do and if they don't like it, then too bad. What's slippery is allowing the teacher or school to decide what is appropriate without notifying the parents.
This is a bit of fear-mongering. If "enough parents" have an issue with the curriculum, then we need to consider that there may actually be a problem with the curriculum. However, that is not the issue in this case. The school board didn't have an issue with the curriculum. They had an issue with how the principal handled it.
I don't think it's tough at all. You notify parents if you are dealing with anything that would be construed as sexual in nature. If you're going to be showing genitalia, sex acts or suggestive sexual acts, you notify parents. If they have an issue with it then they bring it up with the teacher. The teacher should be prepared to make accommodations for that student, such as sitting in with another class or having an excused absence.
Someone did an interview with the school board member & he stated that this wasn't the only issue with her. He eluded that there were ongoing issues that she was well aware of, despite her saying that she had no idea. Here is the full interview: