The child is a minor, the school does not have legal authority to separate the parents and the children with a confidentiality document, because by law the parents represent the child's interests.
It is also the parents home and internet connection. Their property. The school has no jurisdiction over that, and needs to stop making threats in that regard.
Someone or many someones needs to call their lawyer(s) to give the teachers union a verbal warning of the legal cowpie they are stepping in.