What can be seen or not seen when the kids join a remote classroom clearly falls entirely into the parents' responsibility. You don't send your kid to school naked either. If I have to do a Zoom meeting or video conversation, I routinely double-check that the background is neutral.
Once it is being broadcast, of course anything can be reported to authorities, by anyone who sees it; it's no different from someone watching you through your living room window as you strangle somebody
The sticking point, which I think will indeed need to be settled in court eventually, is how far school rules can be enforced off school grounds. There is precedent; teachers have been fired and students suspended for drinking privately with host families after hours on school-sponsored foreign trips.