Yup. I bet it's really because of the teachers union and has NOTHING to do with student privacy.
You'd lose that bet.
The school absolutely has an obligation to protect the academic privacy of students who are normally within a well-established forum, the classroom.
Putting children in the home and essentially broadcasting their behaviors into private homes is a whole new ballgame with no rules established.
The school district tried the easiest way out: make the parents sign what is essentially a form of NDA.
I think they will, and should, fail.
Schools have been protected from the Online revolution by and large, which is a huge inconsistency with what everyone has been dealing with. No doubt more and more classrooms will be virtual, so policies, legal precedents, and laws will have to be established to govern this model.
The School District is simply taking the first step: "Welp, we asked people to not look in to our classrooms, so it's not our fault that Parent X put a video of your stuttering child on Facebook and it went viral".
That's all this is, the School District's #1 priority has always been protecting itself from liability. That's not actually bad, we have to form some type of convention to reasonably teach large numbers of children while at the same time protecting them from being spied upon by parents and preverts.
Overall this is fully expected and should be done.